fix(buffer): avoid shared _extendedAttrs sentinel breaking addon-image

EMPTY_SPARSE_EXTENDED was shared across lines via copyFrom; addon-image
writes _extendedAttrs directly without copy-on-write, which caused tile
eviction failures and broke instanceof Object checks.

Use undefined for empty extended maps (keep EMPTY_SPARSE_MAP for _combined).
Ensure addon-image forks the map before writes and update the accessor test.

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

.github/workflows/ci.yml

@@ -11,9 +11,9 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v5
     - name: Use Node.js 22.x
-      uses: actions/setup-node@v3
+      uses: actions/setup-node@v5
       with:
         node-version: 22.x
         cache: 'npm'
@@ -69,7 +69,7 @@ jobs:
           ./addons/addon-webgl/out/* \
           ./addons/addon-webgl/out-*st/*
     - name: Upload artifacts
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: build-artifacts
         path: compressed-build.zip
@@ -79,9 +79,9 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v5
     - name: Use Node.js 22.x
-      uses: actions/setup-node@v3
+      uses: actions/setup-node@v5
       with:
         node-version: 22.x
         cache: 'npm'
@@ -100,16 +100,16 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v5
     - name: Use Node.js 22.x
-      uses: actions/setup-node@v3
+      uses: actions/setup-node@v5
       with:
         node-version: 22.x
         cache: 'npm'
     - name: Install dependencies
       run: |
         npm ci
-    - uses: actions/download-artifact@v4
+    - uses: actions/download-artifact@v7
       with:
         name: build-artifacts
     - name: Unzip artifacts
@@ -129,6 +129,7 @@ jobs:
         exit $EXIT_CODE
 
   test-unit:
+    needs: build
     timeout-minutes: 20
     strategy:
       matrix:
@@ -136,21 +137,16 @@ jobs:
         runs-on: [ubuntu, macos, windows]
     runs-on: ${{ matrix.runs-on }}-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v5
     - name: Use Node.js ${{ matrix.node-version }}.x
-      uses: actions/setup-node@v3
+      uses: actions/setup-node@v5
       with:
         node-version: ${{ matrix.node-version }}.x
         cache: 'npm'
     - name: Install dependencies
       run: |
         npm ci
-    - name: Wait for build job
+    - uses: actions/download-artifact@v7
-      uses: NathanFirmo/wait-for-other-job@v1.1.1
-      with:
-        token: ${{ secrets.GITHUB_TOKEN }}
-        job: build
-    - uses: actions/download-artifact@v4
       with:
         name: build-artifacts
     - name: Unzip artifacts
@@ -166,6 +162,7 @@ jobs:
       run: npm run test-unit --forbid-only
 
   test-integration:
+    needs: build
     timeout-minutes: 20
     strategy:
       matrix:
@@ -174,9 +171,9 @@ jobs:
         browser: [chromium, firefox, webkit]
     runs-on: ${{ matrix.runs-on }}
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v5
     - name: Use Node.js ${{ matrix.node-version }}.x
-      uses: actions/setup-node@v3
+      uses: actions/setup-node@v5
       with:
         node-version: ${{ matrix.node-version }}.x
         cache: 'npm'
@@ -185,12 +182,7 @@ jobs:
         npm ci
     - name: Install playwright
       run: npx playwright install --with-deps ${{ matrix.browser }}
-    - name: Wait for build job
+    - uses: actions/download-artifact@v7
-      uses: NathanFirmo/wait-for-other-job@v1.1.1
-      with:
-        token: ${{ secrets.GITHUB_TOKEN }}
-        job: build
-    - uses: actions/download-artifact@v4
       with:
         name: build-artifacts
     - name: Unzip artifacts
@@ -240,9 +232,9 @@ jobs:
       matrix:
         node-version: [22]
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v5
     - name: Use Node.js ${{ matrix.node-version }}.x
-      uses: actions/setup-node@v3
+      uses: actions/setup-node@v5
       with:
         node-version: ${{ matrix.node-version }}.x
         cache: 'npm'
@@ -251,7 +243,7 @@ jobs:
         npm ci
     - name: Install playwright
       run: npx playwright install
-    - uses: actions/download-artifact@v4
+    - uses: actions/download-artifact@v7
       with:
         name: build-artifacts
     - name: Unzip artifacts

.github/workflows/codeql-analysis.yml

@@ -24,12 +24,12 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v3
+      uses: actions/checkout@v5
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@v2
+      uses: github/codeql-action/init@v4
       with:
         languages: ${{ matrix.language }}
     - name: Autobuild
-      uses: github/codeql-action/autobuild@v2
+      uses: github/codeql-action/autobuild@v4
     - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@v2
+      uses: github/codeql-action/analyze@v4

.github/workflows/copilot-setup-steps.yml

@@ -18,9 +18,9 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
       - name: Use Node.js 22.x
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v5
         with:
           node-version: 22.x
           cache: 'npm'

.github/workflows/release.yml

@@ -10,9 +10,9 @@ jobs:
   release:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v5
     - name: Use Node.js 22.x
-      uses: actions/setup-node@v3
+      uses: actions/setup-node@v5
       with:
         node-version: 22.x
         cache: 'npm'

addons/addon-clipboard/src/ClipboardAddon.ts

@@ -78,13 +78,43 @@ export class BrowserClipboardProvider implements IClipboardProvider {
   }
 }
 
+/**
+ * TODO: Once the base64 handling on Uint8Arrays is more common,
+ * remove the btoa/atob fallbacks below.
+ */
+interface IUint8ArrayB64 extends Uint8Array {
+  toBase64(): string;
+}
+interface IUint8ArrayB64Ctor extends Uint8ArrayConstructor {
+  fromBase64(s: string): IUint8ArrayB64;
+}
+
 export class Base64 implements IBase64 {
   public encodeText(data: string): string {
-    return btoa(data);
+    const bytes = new TextEncoder().encode(data) as IUint8ArrayB64;
+    if (bytes.toBase64 !== undefined) {
+      return bytes.toBase64();
+    }
+    let bin = '';
+    for (let i = 0; i < bytes.length; i++) {
+      bin += String.fromCharCode(bytes[i]);
+    }
+    return btoa(bin);
   }
   public decodeText(data: string): string {
+    if ((Uint8Array as IUint8ArrayB64Ctor).fromBase64 !== undefined) {
+      try {
+        return new TextDecoder().decode((Uint8Array as IUint8ArrayB64Ctor).fromBase64(data));
+      } catch {}
+      return '';
+    }
     try {
-      return atob(data);
+      const bin = atob(data);
+      const bytes = new Uint8Array(bin.length);
+      for (let i = 0; i < bin.length; ++i) {
+        bytes[i] = bin.charCodeAt(i);
+      }
+      return new TextDecoder().decode(bytes);
     } catch {}
     return '';
   }

addons/addon-clipboard/test/ClipboardAddon.test.ts

@@ -29,7 +29,7 @@ test.describe('ClipboardAddon', () => {
       await ctx.page.context().grantPermissions(['clipboard-read', 'clipboard-write']);
     }
     await ctx.page.evaluate(`
-      window.term.reset()
+      window.term.reset();
       window.clipboard?.dispose();
       window.clipboard = new ClipboardAddon();
       window.term.loadAddon(window.clipboard);
@@ -92,4 +92,22 @@ test.describe('ClipboardAddon', () => {
       deepEqual(await ctx.page.evaluate(() => window.navigator.clipboard.readText()), '');
     });
   });
+
+  test.describe('non-ASCII', () => {
+    const testDataEncoded = '4oKsbWzDpMO8dMOf';
+    const testDataDecoded = '€mläütß';
+    test('write simple string', async () => {
+      await ctx.proxy.write(`\x1b]52;c;${testDataEncoded}\x07`);
+      deepEqual(await ctx.page.evaluate(() => window.navigator.clipboard.readText()), testDataDecoded);
+    });
+    test('read simple string', async () => {
+      await ctx.page.evaluate(`
+        window.data2 = [];
+        window.term.onData(e => data2.push(e));
+      `);
+      await ctx.page.evaluate((d) => window.navigator.clipboard.writeText(d), testDataDecoded);
+      await ctx.proxy.write(`\x1b]52;c;?\x07`);
+      deepEqual(await ctx.page.evaluate('window.data2'), [`\x1b]52;c;${testDataEncoded}\x07`]);
+    });
+  });
 });

addons/addon-image/src/ImageStorage.ts

@@ -436,9 +436,9 @@ export class ImageStorage implements IDisposable {
       for (let col = 0; col < cols; ++col) {
         let e: IExtendedAttrsImage;
         if (line.getBg(col) & BgFlags.HAS_EXTENDED) {
-          e = line._extendedAttrs[col] ?? EMPTY_ATTRS;
+          e = line._extendedAttrs?.[col] ?? EMPTY_ATTRS;
         } else {
-          const maybeImg = line._extendedAttrs[col] as IExtendedAttrsImage | undefined;
+          const maybeImg = line._extendedAttrs?.[col] as IExtendedAttrsImage | undefined;
           if (!maybeImg || maybeImg.imageId === undefined || maybeImg.imageId === -1) {
             continue;
           }
@@ -461,7 +461,7 @@ export class ImageStorage implements IDisposable {
            * Also check _extendedAttrs directly for cells where text cleared HAS_EXTENDED.
            */
           while (++col < cols) {
-            const nextE = line._extendedAttrs[col] as IExtendedAttrsImage | undefined;
+            const nextE = line._extendedAttrs?.[col] as IExtendedAttrsImage | undefined;
             if (!nextE || nextE.imageId !== imageId || nextE.tileId !== startTile + count) {
               break;
             }
@@ -515,7 +515,7 @@ export class ImageStorage implements IDisposable {
     for (let row = 0; row < rows; ++row) {
       const line = buffer.lines.get(row) as IBufferLineExt;
       if (line.getBg(oldCol) & BgFlags.HAS_EXTENDED) {
-        const e: IExtendedAttrsImage = line._extendedAttrs[oldCol] ?? EMPTY_ATTRS;
+        const e: IExtendedAttrsImage = line._extendedAttrs?.[oldCol] ?? EMPTY_ATTRS;
         const imageId = e.imageId;
         if (imageId === undefined || imageId === -1) {
           continue;
@@ -560,7 +560,7 @@ export class ImageStorage implements IDisposable {
     const buffer = this._terminal._core.buffer;
     const line = buffer.lines.get(y) as IBufferLineExt;
     if (line && line.getBg(x) & BgFlags.HAS_EXTENDED) {
-      const e: IExtendedAttrsImage = line._extendedAttrs[x] ?? EMPTY_ATTRS;
+      const e: IExtendedAttrsImage = line._extendedAttrs?.[x] ?? EMPTY_ATTRS;
       if (e.imageId && e.imageId !== -1) {
         const orig = this._images.get(e.imageId)?.orig;
         if (window.ImageBitmap && orig instanceof ImageBitmap) {
@@ -580,7 +580,7 @@ export class ImageStorage implements IDisposable {
     const buffer = this._terminal._core.buffer;
     const line = buffer.lines.get(y) as IBufferLineExt;
     if (line && line.getBg(x) & BgFlags.HAS_EXTENDED) {
-      const e: IExtendedAttrsImage = line._extendedAttrs[x] ?? EMPTY_ATTRS;
+      const e: IExtendedAttrsImage = line._extendedAttrs?.[x] ?? EMPTY_ATTRS;
       if (e.imageId && e.imageId !== -1 && e.tileId !== -1) {
         const spec = this._images.get(e.imageId);
         if (spec) {
@@ -611,7 +611,7 @@ export class ImageStorage implements IDisposable {
 
   private _writeToCell(line: IBufferLineExt, x: number, imageId: number, tileId: number): void {
     if (line._data[x * Cell.SIZE + Cell.BG] & BgFlags.HAS_EXTENDED) {
-      const old = line._extendedAttrs[x];
+      const old = line._extendedAttrs?.[x];
       if (old) {
         if (old.imageId !== undefined) {
           // found an old ExtendedAttrsImage, since we know that
@@ -627,13 +627,13 @@ export class ImageStorage implements IDisposable {
           return;
         }
         // found a plain ExtendedAttrs instance, clone it to new entry
-        line._extendedAttrs[x] = new ExtendedAttrsImage(old.ext, old.urlId, imageId, tileId);
+        (line._extendedAttrs ??= {})[x] = new ExtendedAttrsImage(old.ext, old.urlId, imageId, tileId);
         return;
       }
     }
     // fall-through: always create new ExtendedAttrsImage entry
     line._data[x * Cell.SIZE + Cell.BG] |= BgFlags.HAS_EXTENDED;
-    line._extendedAttrs[x] = new ExtendedAttrsImage(0, 0, imageId, tileId);
+    (line._extendedAttrs ??= {})[x] = new ExtendedAttrsImage(0, 0, imageId, tileId);
   }
 
   private _evictOnAlternate(): void {
@@ -652,7 +652,7 @@ export class ImageStorage implements IDisposable {
       }
       for (let x = 0; x < this._terminal.cols; ++x) {
         if (line._data[x * Cell.SIZE + Cell.BG] & BgFlags.HAS_EXTENDED) {
-          const imgId = line._extendedAttrs[x]?.imageId;
+          const imgId = line._extendedAttrs?.[x]?.imageId;
           if (imgId) {
             const spec = this._images.get(imgId);
             if (spec) {

addons/addon-image/src/Types.ts

@@ -64,7 +64,7 @@ export interface IExtendedAttrsImage extends IExtendedAttrs {
 
 /* eslint-disable */
 export interface IBufferLineExt extends IBufferLine {
-  _extendedAttrs: {[index: number]: IExtendedAttrsImage | undefined};
+  _extendedAttrs: {[index: number]: IExtendedAttrsImage | undefined} | undefined;
   _data: Uint32Array;
 }
 

addons/addon-image/test/ImageAddon.test.ts

@@ -118,7 +118,10 @@ test.describe('ImageAddon', () => {
     }
     // bufferline privates
     strictEqual(await ctx.page.evaluate('window.term._core.buffer.lines.get(0)._data instanceof Uint32Array'), true);
-    strictEqual(await ctx.page.evaluate('window.term._core.buffer.lines.get(0)._extendedAttrs instanceof Object'), true);
+    strictEqual(await ctx.page.evaluate(`(() => {
+      const ea = window.term._core.buffer.lines.get(0)._extendedAttrs;
+      return ea === undefined || (typeof ea === 'object' && ea !== null);
+    })()`), true);
     // inputhandler privates
     strictEqual(await ctx.page.evaluate('window.term._core._inputHandler._curAttrData.constructor.name'), '_AttributeData');
     strictEqual(await ctx.page.evaluate('window.term._core._inputHandler._parser.constructor.name'), 'EscapeSequenceParser');

addons/addon-webgl/src/GlyphRenderer.ts

@@ -389,8 +389,9 @@ export class GlyphRenderer extends Disposable {
     gl.bindTexture(gl.TEXTURE_2D, this._atlasTextures[i].texture);
     gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
     gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
+    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
+    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
     gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, atlas.pages[i].canvas);
-    gl.generateMipmap(gl.TEXTURE_2D);
     this._atlasTextures[i].version = atlas.pages[i].version;
   }
 

addons/addon-webgl/src/TextureAtlas.ts

@@ -1080,17 +1080,33 @@ class AtlasPage {
     size: number,
     sourcePages?: AtlasPage[]
   ) {
-    if (sourcePages) {
-      for (const p of sourcePages) {
-        this._glyphs.push(...p.glyphs);
-        this._usedPixels += p._usedPixels;
-      }
-    }
     this.canvas = createCanvas(document, size, size);
     // The canvas needs alpha because we use clearColor to convert the background color to alpha.
     // It might also contain some characters with transparent backgrounds if allowTransparency is
     // set.
     this.ctx = throwIfFalsy(this.canvas.getContext('2d', { alpha: true }));
+    if (sourcePages) {
+      if (sourcePages.length === 4) {
+        // optimized for quadmerge
+        this._glyphs = this._glyphs.concat(
+          sourcePages[0].glyphs,
+          sourcePages[1].glyphs,
+          sourcePages[2].glyphs,
+          sourcePages[3].glyphs
+        );
+        this._usedPixels = sourcePages[0]._usedPixels +
+          sourcePages[1]._usedPixels +
+          sourcePages[2]._usedPixels +
+          sourcePages[3]._usedPixels;
+      } else {
+        // fallback for non quadmerges (should never be used)
+        for (let i = 0; i < sourcePages.length; ++i) {
+          this._glyphs = this._glyphs.concat(sourcePages[i].glyphs);
+          this._usedPixels += sourcePages[i]._usedPixels;
+        }
+
+      }
+    }
   }
 
   public clear(): void {

addons/addon-webgl/src/WebglRenderer.ts

@@ -456,7 +456,18 @@ export class WebglRenderer extends Disposable implements IRenderer {
 
     for (y = start; y <= end; y++) {
       row = y + terminal.buffer.ydisp;
-      line = terminal.buffer.lines.get(row)!;
+      const bufferLine = terminal.buffer.lines.get(row);
+      if (!bufferLine) {
+        this._model.lineLengths[y] = 0;
+        for (x = 0; x < terminal.cols; x++) {
+          j = ((y * terminal.cols) + x) * RenderModelConstants.INDICIES_PER_CELL;
+          modelUpdated = true;
+          this._nullModelCell(x, y, j, 0, 0, 0);
+        }
+        this._setRowBlinkState(y, false);
+        continue;
+      }
+      line = bufferLine;
       let rowHasBlinkingCells = false;
       this._model.lineLengths[y] = 0;
       isCursorRow = cursorY === row;
@@ -587,13 +598,9 @@ export class WebglRenderer extends Disposable implements IRenderer {
           // Null out non-first cells
           for (x++; x <= lastCharX; x++) {
             j = ((y * terminal.cols) + x) * RenderModelConstants.INDICIES_PER_CELL;
-            this._glyphRenderer.value!.updateCell(x, y, NULL_CELL_CODE, 0, 0, 0, NULL_CELL_CHAR, 0, 0);
-            this._model.cells[j] = NULL_CELL_CODE;
             // Don't re-resolve the cell color since multi-colored ligature backgrounds are not
             // supported
-            this._model.cells[j + RenderModelConstants.BG_OFFSET] = this._cellColorResolver.result.bg;
+            this._nullModelCell(x, y, j, this._cellColorResolver.result.bg, this._cellColorResolver.result.fg, this._cellColorResolver.result.ext);
-            this._model.cells[j + RenderModelConstants.FG_OFFSET] = this._cellColorResolver.result.fg;
-            this._model.cells[j + RenderModelConstants.EXT_OFFSET] = this._cellColorResolver.result.ext;
           }
           x--; // Go back to the previous update cell for next iteration
         }
@@ -607,6 +614,14 @@ export class WebglRenderer extends Disposable implements IRenderer {
     this._updateTextBlinkState();
   }
 
+  private _nullModelCell(x: number, y: number, cellIndex: number, bg: number, fg: number, ext: number): void {
+    this._glyphRenderer.value!.updateCell(x, y, NULL_CELL_CODE, bg, fg, ext, NULL_CELL_CHAR, 0, 0);
+    this._model.cells[cellIndex] = NULL_CELL_CODE;
+    this._model.cells[cellIndex + RenderModelConstants.BG_OFFSET] = bg;
+    this._model.cells[cellIndex + RenderModelConstants.FG_OFFSET] = fg;
+    this._model.cells[cellIndex + RenderModelConstants.EXT_OFFSET] = ext;
+  }
+
   private _resetBlinkingRowState(): void {
     this._rowHasBlinkingCells = new Array(this._terminal.rows).fill(false);
     this._rowHasBlinkingCellsCount = 0;

demo/client/components/window/webglWindow.ts

@@ -11,8 +11,33 @@ export class WebglWindow extends BaseWindow implements IControlWindow {
   public readonly label = 'webgl';
 
   private _textureAtlasContainer!: HTMLElement;
+  private _stressRunning = false;
+  private _stressScrolling: HTMLInputElement | undefined;
 
   public build(container: HTMLElement): void {
+    const stressDiv = document.createElement('div');
+    const stressStart = document.createElement('button');
+    stressStart.id = 'stress-start';
+    stressStart.textContent = 'atlas stress start';
+    stressStart.addEventListener('click', () => this._stress());
+    stressDiv.appendChild(stressStart);
+
+    const stressStop = document.createElement('button');
+    stressStop.id = 'stress-stop';
+    stressStop.textContent = 'atlas stress stop';
+    stressStop.addEventListener('click', () => this._stressRunning = false);
+    stressDiv.appendChild(stressStop);
+
+    this._stressScrolling = document.createElement('input');
+    this._stressScrolling.type = 'checkbox';
+    this._stressScrolling.id = 'stress-scrolling';
+    stressDiv.appendChild(this._stressScrolling);
+    const stressScrollingLabel = document.createElement('label');
+    stressScrollingLabel.htmlFor = 'stress-scrolling';
+    stressScrollingLabel.textContent = 'stress scrolling';
+    stressDiv.appendChild(stressScrollingLabel);
+    container.appendChild(stressDiv);
+
     const zoomCheckbox = document.createElement('input');
     zoomCheckbox.type = 'checkbox';
     zoomCheckbox.id = 'texture-atlas-zoom';
@@ -48,4 +73,33 @@ export class WebglWindow extends BaseWindow implements IControlWindow {
     canvas.style.width = `${canvas.width / dpr}px`;
     canvas.style.height = `${canvas.height / dpr}px`;
   }
+
+  private async _stress(): Promise<void> {
+    const TEXT = '!"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~';
+    if (this._stressRunning) {
+      return;
+    }
+    this._stressRunning = true;
+    const scrolling = !!(this._stressScrolling?.checked);
+    for (let r = 0; r < 256; r += 2) {
+      for (let g = 0; g < 256; g += 2) {
+        let s: string[] = [];
+        for (let b = 0; b < 256; b += 2) {
+          if (!this._stressRunning) {
+            return;
+          }
+          const rbg = `RGB: ${[r, g, b]}`;
+          s.push(`\r\x1b[38;2;${r};${g};${b}m${rbg.padEnd(18, ' ')}${TEXT}`);
+          if (s.length >= 16) {
+            if (scrolling) {
+              await new Promise<void>(r => this._terminal.write(s.join('\r\n') + '\r\n', r));
+            } else {
+              await new Promise<void>(r => this._terminal.write('\x1b[H' + s.join('\r\n'), r));
+            }
+            s = [];
+          }
+        }
+      }
+    }
+  }
 }

package-lock.json

@@ -4700,18 +4700,22 @@
       }
     },
     "node_modules/glob": {
-      "version": "13.0.6",
+      "version": "10.5.0",
-      "resolved": "https://registry.npmjs.org/glob/-/glob-13.0.6.tgz",
+      "resolved": "https://registry.npmjs.org/glob/-/glob-10.5.0.tgz",
-      "integrity": "sha512-Wjlyrolmm8uDpm/ogGyXZXb1Z+Ca2B8NbJwqBVg0axK9GbBeoS7yGV6vjXnYdGm6X53iehEuxxbyiKp8QmN4Vw==",
+      "integrity": "sha512-DfXN8DfhJ7NH3Oe7cFmu3NCu1wKbkReJ8TorzSAFbSKrlNaQSKfIzqYqVY8zlbs2NLBbWpRiU52GX2PbaBVNkg==",
+      "deprecated": "Old versions of glob are not supported, and contain widely publicized security vulnerabilities, which have been fixed in the current version. Please update. Support for old versions may be purchased (at exorbitant rates) by contacting i@izs.me",
       "dev": true,
-      "license": "BlueOak-1.0.0",
+      "license": "ISC",
       "dependencies": {
-        "minimatch": "^10.2.2",
+        "foreground-child": "^3.1.0",
-        "minipass": "^7.1.3",
+        "jackspeak": "^3.1.2",
-        "path-scurry": "^2.0.2"
+        "minimatch": "^9.0.4",
+        "minipass": "^7.1.2",
+        "package-json-from-dist": "^1.0.0",
+        "path-scurry": "^1.11.1"
       },
-      "engines": {
+      "bin": {
-        "node": "18 || 20 || >=22"
+        "glob": "dist/esm/bin.mjs"
       },
       "funding": {
         "url": "https://github.com/sponsors/isaacs"
@@ -4737,6 +4741,39 @@
       "dev": true,
       "license": "BSD-2-Clause"
     },
+    "node_modules/glob/node_modules/balanced-match": {
+      "version": "1.0.2",
+      "resolved": "https://registry.npmjs.org/balanced-match/-/balanced-match-1.0.2.tgz",
+      "integrity": "sha512-3oSeUO0TMV67hN1AmbXsK4yaqU7tjiHlbxRDZOpH0KW9+CeX4bRAaX0Anxt0tx2MrpRpWwQaPwIlISEJhYU5Pw==",
+      "dev": true,
+      "license": "MIT"
+    },
+    "node_modules/glob/node_modules/brace-expansion": {
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/brace-expansion/-/brace-expansion-2.1.0.tgz",
+      "integrity": "sha512-TN1kCZAgdgweJhWWpgKYrQaMNHcDULHkWwQIspdtjV4Y5aurRdZpjAqn6yX3FPqTA9ngHCc4hJxMAMgGfve85w==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "balanced-match": "^1.0.0"
+      }
+    },
+    "node_modules/glob/node_modules/minimatch": {
+      "version": "9.0.9",
+      "resolved": "https://registry.npmjs.org/minimatch/-/minimatch-9.0.9.tgz",
+      "integrity": "sha512-OBwBN9AL4dqmETlpS2zasx+vTeWclWzkblfZk7KTA5j3jeOONz/tRCnZomUyvNg83wL5Zv9Ss6HMJXAgL8R2Yg==",
+      "dev": true,
+      "license": "ISC",
+      "dependencies": {
+        "brace-expansion": "^2.0.2"
+      },
+      "engines": {
+        "node": ">=16 || 14 >=14.17"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/isaacs"
+      }
+    },
     "node_modules/gopd": {
       "version": "1.2.0",
       "resolved": "https://registry.npmjs.org/gopd/-/gopd-1.2.0.tgz",
@@ -5758,44 +5795,15 @@
       "license": "MIT"
     },
     "node_modules/mocha/node_modules/brace-expansion": {
-      "version": "2.1.1",
+      "version": "2.1.0",
-      "resolved": "https://registry.npmjs.org/brace-expansion/-/brace-expansion-2.1.1.tgz",
+      "resolved": "https://registry.npmjs.org/brace-expansion/-/brace-expansion-2.1.0.tgz",
-      "integrity": "sha512-WR1cURNjuvBLMZBMbqM0UoE+WAfdUcEV1ccD8PVBVOI+Z3ND4+SZbN8RsfT2bMuG1qwz5RFvPukSZm5fF2D5eA==",
+      "integrity": "sha512-TN1kCZAgdgweJhWWpgKYrQaMNHcDULHkWwQIspdtjV4Y5aurRdZpjAqn6yX3FPqTA9ngHCc4hJxMAMgGfve85w==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
         "balanced-match": "^1.0.0"
       }
     },
-    "node_modules/mocha/node_modules/glob": {
-      "version": "10.5.0",
-      "resolved": "https://registry.npmjs.org/glob/-/glob-10.5.0.tgz",
-      "integrity": "sha512-DfXN8DfhJ7NH3Oe7cFmu3NCu1wKbkReJ8TorzSAFbSKrlNaQSKfIzqYqVY8zlbs2NLBbWpRiU52GX2PbaBVNkg==",
-      "deprecated": "Old versions of glob are not supported, and contain widely publicized security vulnerabilities, which have been fixed in the current version. Please update. Support for old versions may be purchased (at exorbitant rates) by contacting i@izs.me",
-      "dev": true,
-      "license": "ISC",
-      "dependencies": {
-        "foreground-child": "^3.1.0",
-        "jackspeak": "^3.1.2",
-        "minimatch": "^9.0.4",
-        "minipass": "^7.1.2",
-        "package-json-from-dist": "^1.0.0",
-        "path-scurry": "^1.11.1"
-      },
-      "bin": {
-        "glob": "dist/esm/bin.mjs"
-      },
-      "funding": {
-        "url": "https://github.com/sponsors/isaacs"
-      }
-    },
-    "node_modules/mocha/node_modules/lru-cache": {
-      "version": "10.4.3",
-      "resolved": "https://registry.npmjs.org/lru-cache/-/lru-cache-10.4.3.tgz",
-      "integrity": "sha512-JNAzZcXrCt42VGLuYz0zfAzDfAvJWW6AfYlDBQyDV5DClI2m5sAmK+OIO7s59XfsRsWHp02jAJrRadPRGTt6SQ==",
-      "dev": true,
-      "license": "ISC"
-    },
     "node_modules/mocha/node_modules/minimatch": {
       "version": "9.0.9",
       "resolved": "https://registry.npmjs.org/minimatch/-/minimatch-9.0.9.tgz",
@@ -5812,23 +5820,6 @@
         "url": "https://github.com/sponsors/isaacs"
       }
     },
-    "node_modules/mocha/node_modules/path-scurry": {
-      "version": "1.11.1",
-      "resolved": "https://registry.npmjs.org/path-scurry/-/path-scurry-1.11.1.tgz",
-      "integrity": "sha512-Xa4Nw17FS9ApQFJ9umLiJS4orGjm7ZzwUrwamcGQuHSzDyth9boKDaycYdDcZDuqYATXw4HFXgaqWTctW/v1HA==",
-      "dev": true,
-      "license": "BlueOak-1.0.0",
-      "dependencies": {
-        "lru-cache": "^10.2.0",
-        "minipass": "^5.0.0 || ^6.0.2 || ^7.0.0"
-      },
-      "engines": {
-        "node": ">=16 || 14 >=14.18"
-      },
-      "funding": {
-        "url": "https://github.com/sponsors/isaacs"
-      }
-    },
     "node_modules/ms": {
       "version": "2.1.3",
       "resolved": "https://registry.npmjs.org/ms/-/ms-2.1.3.tgz",
@@ -5996,6 +5987,24 @@
         "node": ">=8"
       }
     },
+    "node_modules/nyc/node_modules/glob": {
+      "version": "13.0.6",
+      "resolved": "https://registry.npmjs.org/glob/-/glob-13.0.6.tgz",
+      "integrity": "sha512-Wjlyrolmm8uDpm/ogGyXZXb1Z+Ca2B8NbJwqBVg0axK9GbBeoS7yGV6vjXnYdGm6X53iehEuxxbyiKp8QmN4Vw==",
+      "dev": true,
+      "license": "BlueOak-1.0.0",
+      "dependencies": {
+        "minimatch": "^10.2.2",
+        "minipass": "^7.1.3",
+        "path-scurry": "^2.0.2"
+      },
+      "engines": {
+        "node": "18 || 20 || >=22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/isaacs"
+      }
+    },
     "node_modules/nyc/node_modules/locate-path": {
       "version": "5.0.0",
       "resolved": "https://registry.npmjs.org/locate-path/-/locate-path-5.0.0.tgz",
@@ -6038,6 +6047,23 @@
         "node": ">=8"
       }
     },
+    "node_modules/nyc/node_modules/path-scurry": {
+      "version": "2.0.2",
+      "resolved": "https://registry.npmjs.org/path-scurry/-/path-scurry-2.0.2.tgz",
+      "integrity": "sha512-3O/iVVsJAPsOnpwWIeD+d6z/7PmqApyQePUtCndjatj/9I5LylHvt5qluFaBT3I5h3r1ejfR056c+FCv+NnNXg==",
+      "dev": true,
+      "license": "BlueOak-1.0.0",
+      "dependencies": {
+        "lru-cache": "^11.0.0",
+        "minipass": "^7.1.2"
+      },
+      "engines": {
+        "node": "18 || 20 || >=22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/isaacs"
+      }
+    },
     "node_modules/nyc/node_modules/signal-exit": {
       "version": "3.0.7",
       "resolved": "https://registry.npmjs.org/signal-exit/-/signal-exit-3.0.7.tgz",
@@ -6435,22 +6461,29 @@
       "license": "MIT"
     },
     "node_modules/path-scurry": {
-      "version": "2.0.2",
+      "version": "1.11.1",
-      "resolved": "https://registry.npmjs.org/path-scurry/-/path-scurry-2.0.2.tgz",
+      "resolved": "https://registry.npmjs.org/path-scurry/-/path-scurry-1.11.1.tgz",
-      "integrity": "sha512-3O/iVVsJAPsOnpwWIeD+d6z/7PmqApyQePUtCndjatj/9I5LylHvt5qluFaBT3I5h3r1ejfR056c+FCv+NnNXg==",
+      "integrity": "sha512-Xa4Nw17FS9ApQFJ9umLiJS4orGjm7ZzwUrwamcGQuHSzDyth9boKDaycYdDcZDuqYATXw4HFXgaqWTctW/v1HA==",
       "dev": true,
       "license": "BlueOak-1.0.0",
       "dependencies": {
-        "lru-cache": "^11.0.0",
+        "lru-cache": "^10.2.0",
-        "minipass": "^7.1.2"
+        "minipass": "^5.0.0 || ^6.0.2 || ^7.0.0"
       },
       "engines": {
-        "node": "18 || 20 || >=22"
+        "node": ">=16 || 14 >=14.18"
       },
       "funding": {
         "url": "https://github.com/sponsors/isaacs"
       }
     },
+    "node_modules/path-scurry/node_modules/lru-cache": {
+      "version": "10.4.3",
+      "resolved": "https://registry.npmjs.org/lru-cache/-/lru-cache-10.4.3.tgz",
+      "integrity": "sha512-JNAzZcXrCt42VGLuYz0zfAzDfAvJWW6AfYlDBQyDV5DClI2m5sAmK+OIO7s59XfsRsWHp02jAJrRadPRGTt6SQ==",
+      "dev": true,
+      "license": "ISC"
+    },
     "node_modules/path-to-regexp": {
       "version": "8.4.2",
       "resolved": "https://registry.npmjs.org/path-to-regexp/-/path-to-regexp-8.4.2.tgz",
@@ -6827,6 +6860,41 @@
         "url": "https://github.com/sponsors/isaacs"
       }
     },
+    "node_modules/rimraf/node_modules/glob": {
+      "version": "13.0.6",
+      "resolved": "https://registry.npmjs.org/glob/-/glob-13.0.6.tgz",
+      "integrity": "sha512-Wjlyrolmm8uDpm/ogGyXZXb1Z+Ca2B8NbJwqBVg0axK9GbBeoS7yGV6vjXnYdGm6X53iehEuxxbyiKp8QmN4Vw==",
+      "dev": true,
+      "license": "BlueOak-1.0.0",
+      "dependencies": {
+        "minimatch": "^10.2.2",
+        "minipass": "^7.1.3",
+        "path-scurry": "^2.0.2"
+      },
+      "engines": {
+        "node": "18 || 20 || >=22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/isaacs"
+      }
+    },
+    "node_modules/rimraf/node_modules/path-scurry": {
+      "version": "2.0.2",
+      "resolved": "https://registry.npmjs.org/path-scurry/-/path-scurry-2.0.2.tgz",
+      "integrity": "sha512-3O/iVVsJAPsOnpwWIeD+d6z/7PmqApyQePUtCndjatj/9I5LylHvt5qluFaBT3I5h3r1ejfR056c+FCv+NnNXg==",
+      "dev": true,
+      "license": "BlueOak-1.0.0",
+      "dependencies": {
+        "lru-cache": "^11.0.0",
+        "minipass": "^7.1.2"
+      },
+      "engines": {
+        "node": "18 || 20 || >=22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/isaacs"
+      }
+    },
     "node_modules/router": {
       "version": "2.2.0",
       "resolved": "https://registry.npmjs.org/router/-/router-2.2.0.tgz",
@@ -7586,6 +7654,41 @@
         "node": "20 || >=22"
       }
     },
+    "node_modules/test-exclude/node_modules/glob": {
+      "version": "13.0.6",
+      "resolved": "https://registry.npmjs.org/glob/-/glob-13.0.6.tgz",
+      "integrity": "sha512-Wjlyrolmm8uDpm/ogGyXZXb1Z+Ca2B8NbJwqBVg0axK9GbBeoS7yGV6vjXnYdGm6X53iehEuxxbyiKp8QmN4Vw==",
+      "dev": true,
+      "license": "BlueOak-1.0.0",
+      "dependencies": {
+        "minimatch": "^10.2.2",
+        "minipass": "^7.1.3",
+        "path-scurry": "^2.0.2"
+      },
+      "engines": {
+        "node": "18 || 20 || >=22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/isaacs"
+      }
+    },
+    "node_modules/test-exclude/node_modules/path-scurry": {
+      "version": "2.0.2",
+      "resolved": "https://registry.npmjs.org/path-scurry/-/path-scurry-2.0.2.tgz",
+      "integrity": "sha512-3O/iVVsJAPsOnpwWIeD+d6z/7PmqApyQePUtCndjatj/9I5LylHvt5qluFaBT3I5h3r1ejfR056c+FCv+NnNXg==",
+      "dev": true,
+      "license": "BlueOak-1.0.0",
+      "dependencies": {
+        "lru-cache": "^11.0.0",
+        "minipass": "^7.1.2"
+      },
+      "engines": {
+        "node": "18 || 20 || >=22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/isaacs"
+      }
+    },
     "node_modules/tiny-emitter": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/tiny-emitter/-/tiny-emitter-2.1.0.tgz",

package.json

@@ -11,6 +11,9 @@
   "workspaces": [
     "addons/*"
   ],
+  "overrides": {
+    "serialize-javascript": "^7.0.3"
+  },
   "keywords": [
     "cli",
     "command-line",
@@ -115,8 +118,5 @@
     "webpack-cli": "^7.0.2",
     "ws": "^8.20.0",
     "xterm-benchmark": "^0.3.2"
-  },
-  "overrides": {
-    "serialize-javascript": "^7.0.5"
   }
 }

src/browser/CoreBrowserTerminal.ts

@@ -1072,10 +1072,6 @@ export class CoreBrowserTerminal extends CoreTerminal implements ITerminal {
    * Clear the entire buffer, making the prompt line the new first line.
    */
   public clear(): void {
-    if (this.buffer.ybase === 0 && this.buffer.y === 0) {
-      // Don't clear if it's already clear
-      return;
-    }
     this.buffer.clearAllMarkers();
     this.buffer.lines.set(0, this.buffer.lines.get(this.buffer.ybase + this.buffer.y)!);
     this.buffer.lines.length = 1;

src/browser/renderer/dom/DomRenderer.ts

@@ -536,9 +536,14 @@ export class DomRenderer extends Disposable implements IRenderer {
     for (let y = start; y <= end; y++) {
       const row = y + buffer.ydisp;
       const rowElement = this._rowElements[y];
+      if (!rowElement) {
+        continue;
+      }
       const lineData = buffer.lines.get(row);
-      if (!rowElement || !lineData) {
+      if (!lineData) {
-        break;
+        rowElement.replaceChildren();
+        this._setRowBlinkState(y, false);
+        continue;
       }
       rowElement.replaceChildren(
         ...this._rowFactory.createRow(
@@ -610,9 +615,14 @@ export class DomRenderer extends Disposable implements IRenderer {
     for (let i = y; i <= y2; ++i) {
       const row = i + buffer.ydisp;
       const rowElement = this._rowElements[i];
+      if (!rowElement) {
+        continue;
+      }
       const bufferline = buffer.lines.get(row);
-      if (!rowElement || !bufferline) {
+      if (!bufferline) {
-        break;
+        rowElement.replaceChildren();
+        this._setRowBlinkState(i, false);
+        continue;
       }
       rowElement.replaceChildren(
         ...this._rowFactory.createRow(

src/browser/selection/SelectionModel.test.ts

@@ -64,6 +64,13 @@ describe('SelectionModel', () => {
       assert.deepEqual(model.finalSelectionStart, undefined);
       assert.deepEqual(model.finalSelectionEnd, undefined);
     });
+    it('should reset selection start to origin when start row is trimmed', () => {
+      model.selectionStart = [50, 0];
+      model.selectionEnd = [10, 2];
+      assert.equal(model.handleTrim(1), true);
+      assert.deepEqual(model.finalSelectionStart, [0, 0]);
+      assert.deepEqual(model.finalSelectionEnd, [10, 1]);
+    });
   });
 
   describe('finalSelectionStart', () => {

src/browser/selection/SelectionModel.ts

@@ -135,9 +135,10 @@ export class SelectionModel {
       return true;
     }
 
-    // If the selection start is trimmed, ensure the start column is 0.
+    // If the selection start row is trimmed away, reset to the buffer origin.
     if (this.selectionStart && this.selectionStart[1] < 0) {
-      this.selectionStart[1] = 0;
+      this.selectionStart = [0, 0];
+      return true;
     }
     return false;
   }

src/browser/services/CharacterJoinerService.ts

@@ -100,6 +100,7 @@ export class CharacterJoinerService implements ICharacterJoinerService {
 
     const ranges: [number, number][] = [];
     const lineStr = line.translateToString(true);
+    const trimmedLength = line.getTrimmedLength();
 
     // Because some cells can be represented by multiple javascript characters,
     // we track the cell and the string indexes separately. This allows us to
@@ -111,7 +112,7 @@ export class CharacterJoinerService implements ICharacterJoinerService {
     let rangeAttrFG = line.getFg(0);
     let rangeAttrBG = line.getBg(0);
 
-    for (let x = 0; x < line.getTrimmedLength(); x++) {
+    for (let x = 0; x < trimmedLength; x++) {
       line.loadCell(x, this._workCell);
 
       if (this._workCell.getWidth() === 0) {
@@ -147,7 +148,7 @@ export class CharacterJoinerService implements ICharacterJoinerService {
     }
 
     // Process any trailing ranges.
-    if (this._bufferService.cols - rangeStartColumn > 1) {
+    if (trimmedLength - rangeStartColumn > 1) {
       const joinedRanges = this._getJoinedRanges(
         lineStr,
         rangeStartStringIndex,
@@ -216,7 +217,8 @@ export class CharacterJoinerService implements ICharacterJoinerService {
       return;
     }
 
-    for (let x = startCol; x < this._bufferService.cols; x++) {
+    const trimmedLength = line.getTrimmedLength();
+    for (let x = startCol; x < trimmedLength; x++) {
       const width = line.getWidth(x);
       const length = line.getString(x).length || WHITESPACE_CELL_CHAR.length;
 
@@ -264,7 +266,7 @@ export class CharacterJoinerService implements ICharacterJoinerService {
     // If there is still a range left at the end, it must extend all the way to
     // the end of the line.
     if (currentRange) {
-      currentRange[1] = this._bufferService.cols;
+      currentRange[1] = trimmedLength;
     }
   }
 

src/browser/services/SelectionService.ts

@@ -704,7 +704,7 @@ export class SelectionService extends Disposable implements ISelectionService {
         if (this._activeSelectionMode !== SelectionMode.COLUMN) {
           this._model.selectionEnd[0] = this._bufferService.cols;
         }
-        this._model.selectionEnd[1] = Math.min(buffer.ydisp + this._bufferService.rows, buffer.lines.length - 1);
+        this._model.selectionEnd[1] = Math.min(buffer.ydisp + this._bufferService.rows - 1, buffer.lines.length - 1);
       } else {
         if (this._activeSelectionMode !== SelectionMode.COLUMN) {
           this._model.selectionEnd[0] = 0;

src/common/buffer/BufferLine.ts

@@ -71,12 +71,20 @@ export interface IBufferLineStringCache {
  * (if only one particular value is needed) or `loadCell`. For `loadCell` in a loop
  * memory allocs / GC pressure can be greatly reduced by reusing the CellData object.
  */
+/**
+ * Shared empty sparse map for lines with no combining data.
+ * Uses a sentinel + copy-on-write so `copyFrom` can detect empty `_combined` with
+ * reference equality. `_extendedAttrs` uses `undefined` instead (addon-image writes
+ * the map directly without BufferLine APIs).
+ */
+const EMPTY_SPARSE_MAP: {[index: number]: string} = Object.create(null);
+
 export class BufferLine implements IBufferLine {
   protected _data: Uint32Array;
   /** Sparse cache; only read when `IS_COMBINED_MASK` is set in `_data`. */
-  protected _combined: {[index: number]: string} = {};
+  protected _combined: {[index: number]: string} = EMPTY_SPARSE_MAP;
   /** Sparse cache; only read when `HAS_EXTENDED` is set in `_data`. */
-  protected _extendedAttrs: {[index: number]: IExtendedAttrs | undefined} = {};
+  protected _extendedAttrs: {[index: number]: IExtendedAttrs | undefined} | undefined;
   protected _stringCacheEntryRef: WeakRef<IBufferLineStringCacheEntry> | undefined;
   public length: number;
 
@@ -121,6 +129,7 @@ export class BufferLine implements IBufferLine {
     this._invalidateStringCache();
     this._data[index * Constants.CELL_INDICIES + Cell.FG] = value[CHAR_DATA_ATTR_INDEX];
     if (value[CHAR_DATA_CHAR_INDEX].length > 1) {
+      this._ensureCombinedMap();
       this._combined[index] = value[1];
       this._data[index * Constants.CELL_INDICIES + Cell.CONTENT] = index | Content.IS_COMBINED_MASK | (value[CHAR_DATA_WIDTH_INDEX] << Content.WIDTH_SHIFT);
     } else {
@@ -211,7 +220,7 @@ export class BufferLine implements IBufferLine {
       cell.combinedData = '';
     }
     if (cell.bg & BgFlags.HAS_EXTENDED) {
-      cell.extended = this._extendedAttrs[index]!;
+      cell.extended = this._extendedAttrs![index]!;
     } else {
       // Do not mutate cell.extended in place: it may still reference this line's map entry from a
       // prior loadCell into a reused CellData (e.g. $workCell during insert/delete).
@@ -226,10 +235,11 @@ export class BufferLine implements IBufferLine {
   public setCell(index: number, cell: ICellData): void {
     this._invalidateStringCache();
     if (cell.content & Content.IS_COMBINED_MASK) {
+      this._ensureCombinedMap();
       this._combined[index] = cell.combinedData;
     }
     if (cell.bg & BgFlags.HAS_EXTENDED) {
-      this._extendedAttrs[index] = cell.extended;
+      this._ensureExtendedAttrsMap()[index] = cell.extended;
     }
     this._data[index * Constants.CELL_INDICIES + Cell.CONTENT] = cell.content;
     this._data[index * Constants.CELL_INDICIES + Cell.FG] = cell.fg;
@@ -244,7 +254,7 @@ export class BufferLine implements IBufferLine {
   public setCellFromCodepoint(index: number, codePoint: number, width: number, attrs: IAttributeData): void {
     this._invalidateStringCache();
     if (attrs.bg & BgFlags.HAS_EXTENDED) {
-      this._extendedAttrs[index] = attrs.extended;
+      this._ensureExtendedAttrsMap()[index] = attrs.extended;
     }
     this._data[index * Constants.CELL_INDICIES + Cell.CONTENT] = codePoint | (width << Content.WIDTH_SHIFT);
     this._data[index * Constants.CELL_INDICIES + Cell.FG] = attrs.fg;
@@ -262,12 +272,14 @@ export class BufferLine implements IBufferLine {
     let content = this._data[index * Constants.CELL_INDICIES + Cell.CONTENT];
     if (content & Content.IS_COMBINED_MASK) {
       // we already have a combined string, simply add
+      this._ensureCombinedMap();
       this._combined[index] += stringFromCodePoint(codePoint);
     } else {
       if (content & Content.CODEPOINT_MASK) {
         // normal case for combining chars:
         //  - move current leading char + new one into combined string
         //  - set combined flag
+        this._ensureCombinedMap();
         this._combined[index] = stringFromCodePoint(content & Content.CODEPOINT_MASK) + stringFromCodePoint(codePoint);
         content &= ~Content.CODEPOINT_MASK; // set codepoint in buffer to 0
         content |= Content.IS_COMBINED_MASK;
@@ -410,11 +422,13 @@ export class BufferLine implements IBufferLine {
         }
       }
       // remove any cut off extended attributes
-      const extKeys = Object.keys(this._extendedAttrs);
+      if (this._extendedAttrs) {
-      for (let i = 0; i < extKeys.length; i++) {
+        const extKeys = Object.keys(this._extendedAttrs);
-        const key = parseInt(extKeys[i], 10);
+        for (let i = 0; i < extKeys.length; i++) {
-        if (key >= cols) {
+          const key = parseInt(extKeys[i], 10);
-          delete this._extendedAttrs[key];
+          if (key >= cols) {
+            delete this._extendedAttrs[key];
+          }
         }
       }
     }
@@ -450,8 +464,8 @@ export class BufferLine implements IBufferLine {
       }
       return;
     }
-    this._combined = {};
+    this._combined = EMPTY_SPARSE_MAP;
-    this._extendedAttrs = {};
+    this._extendedAttrs = undefined;
     for (let i = 0; i < this.length; ++i) {
       this.setCell(i, fillCellData);
     }
@@ -608,19 +622,57 @@ export class BufferLine implements IBufferLine {
   private _copyCellMapsFrom(src: BufferLine, srcCol: number, destCol: number): void {
     const srcStart = srcCol * Constants.CELL_INDICIES;
     if (src._data[srcStart + Cell.CONTENT] & Content.IS_COMBINED_MASK) {
+      this._ensureCombinedMap();
       this._combined[destCol] = src._combined[srcCol];
     }
     if (src._data[srcStart + Cell.BG] & BgFlags.HAS_EXTENDED) {
-      this._extendedAttrs[destCol] = src._extendedAttrs[srcCol];
+      this._ensureExtendedAttrsMap()[destCol] = src._extendedAttrs![srcCol]!;
     }
   }
 
   /** Rebuild sparse maps from another line, keyed only by `_data` flags. */
   private _copySparseMapsFrom(line: BufferLine): void {
+    const srcCombined = line._combined;
+    const srcExtended = line._extendedAttrs;
+
+    // Fast path: blank / scroll-recycle source (empty `_combined` sentinel, no extended map).
+    if (srcCombined === EMPTY_SPARSE_MAP && !srcExtended) {
+      this._combined = EMPTY_SPARSE_MAP;
+      this._extendedAttrs = undefined;
+      return;
+    }
+    this._copySparseMapsFromDense(line);
+  }
+
+  /** Copy sparse maps by scanning `_data` flags (inlined, no per-cell function calls). */
+  private _copySparseMapsFromDense(line: BufferLine): void {
     this._combined = {};
     this._extendedAttrs = {};
-    for (let i = 0; i < line.length; i++) {
+    const srcData = line._data;
-      this._copyCellMapsFrom(line, i, i);
+    const srcCombined = line._combined;
+    const srcExtended = line._extendedAttrs;
+    const len = line.length;
+    const indices = Constants.CELL_INDICIES;
+    const combined = this._combined;
+    const extendedAttrs = this._extendedAttrs;
+    for (let i = 0; i < len; i++) {
+      const start = i * indices;
+      if (srcData[start + Cell.CONTENT] & Content.IS_COMBINED_MASK) {
+        combined[i] = srcCombined[i];
+      }
+      if (srcData[start + Cell.BG] & BgFlags.HAS_EXTENDED) {
+        extendedAttrs[i] = srcExtended![i];
+      }
     }
   }
+
+  private _ensureCombinedMap(): void {
+    if (this._combined === EMPTY_SPARSE_MAP) {
+      this._combined = {};
+    }
+  }
+
+  private _ensureExtendedAttrsMap(): {[index: number]: IExtendedAttrs | undefined} {
+    return this._extendedAttrs ??= {};
+  }
 }

src/common/input/TextDecoder.test.ts

@@ -79,7 +79,7 @@ function assertDecodedRange(
   }
   const target = new Uint32Array(count);
   const length = decode(input, target);
-  assert.equal(length, count);
+  assert.strictEqual(length, count);
   let mismatchIndex = -1;
   let index = 0;
   for (let i = min; i < max; ++i) {
@@ -92,8 +92,8 @@ function assertDecodedRange(
     }
     index++;
   }
-  assert.equal(mismatchIndex, -1);
+  assert.strictEqual(mismatchIndex, -1);
-  assert.equal(outputToString(target, length), input);
+  assert.strictEqual(outputToString(target, length), input);
 }
 
 const BATCH_SIZE = 8192;
@@ -116,9 +116,9 @@ describe('text encodings', () => {
     const data = new Uint32Array(s.length);
     for (let i = 0; i < s.length; ++i) {
       data[i] = s.charCodeAt(i);
-      assert.equal(stringFromCodePoint(data[i]), s[i]);
+      assert.strictEqual(stringFromCodePoint(data[i]), s[i]);
     }
-    assert.equal(utf32ToString(data), s);
+    assert.strictEqual(utf32ToString(data), s);
   });
 
   describe('StringToUtf32 decoder', () => {
@@ -159,7 +159,7 @@ describe('text encodings', () => {
         const decoder = new StringToUtf32();
         const target = new Uint32Array(5);
         const length = decoder.decode(String.fromCharCode(0xFEFF), target);
-        assert.equal(length, 0);
+        assert.strictEqual(length, 0);
         decoder.clear();
       });
     });
@@ -169,7 +169,7 @@ describe('text encodings', () => {
       const target = new Uint32Array(500);
       for (let i = 0; i < TEST_STRINGS.length; ++i) {
         const length = decoder.decode(TEST_STRINGS[i], target);
-        assert.equal(toString(target, length), TEST_STRINGS[i]);
+        assert.strictEqual(toString(target, length), TEST_STRINGS[i]);
         decoder.clear();
       }
     });
@@ -184,7 +184,7 @@ describe('text encodings', () => {
           const written = decoder.decode(input[i], target);
           decoded += toString(target, written);
         }
-        assert(decoded, 'Ä€𝄞Ö𝄞€Ü𝄞€');
+        assert.strictEqual(decoded, 'Ä€𝄞Ö𝄞€Ü𝄞€');
       });
     });
   });
@@ -226,7 +226,7 @@ describe('text encodings', () => {
         const target = new Uint32Array(5);
         const utf8Data = stringToUtf8Bytes(String.fromCharCode(0xFEFF));
         const length = decoder.decode(utf8Data, target);
-        assert.equal(length, 0);
+        assert.strictEqual(length, 0);
         decoder.clear();
       });
     });
@@ -237,7 +237,7 @@ describe('text encodings', () => {
       for (let i = 0; i < TEST_STRINGS.length; ++i) {
         const utf8Data = stringToUtf8Bytes(TEST_STRINGS[i]);
         const length = decoder.decode(utf8Data, target);
-        assert.equal(toString(target, length), TEST_STRINGS[i]);
+        assert.strictEqual(toString(target, length), TEST_STRINGS[i]);
         decoder.clear();
       }
     });
@@ -252,7 +252,7 @@ describe('text encodings', () => {
           const written = decoder.decode(utf8Data.slice(i, i + 1), target);
           decoded += toString(target, written);
         }
-        assert(decoded, 'ÄÖÜßöäü');
+        assert.strictEqual(decoded, 'ÄÖÜßöäü');
       });
 
       it('2/3 byte sequences - advance by 1', () => {
@@ -264,7 +264,7 @@ describe('text encodings', () => {
           const written = decoder.decode(utf8Data.slice(i, i + 1), target);
           decoded += toString(target, written);
         }
-        assert(decoded, 'Āր܀߀ö€ä€ü');
+        assert.strictEqual(decoded, 'Āր܀߀ö€ä€ü');
       });
 
       it('2/3/4 byte sequences - advance by 1', () => {
@@ -276,7 +276,7 @@ describe('text encodings', () => {
           const written = decoder.decode(utf8Data.slice(i, i + 1), target);
           decoded += toString(target, written);
         }
-        assert(decoded, 'Ä€𝄞Ö𝄞€Ü𝄞€');
+        assert.strictEqual(decoded, 'Ä€𝄞Ö𝄞€Ü𝄞€');
       });
 
       it('2/3/4 byte sequences - advance by 2', () => {
@@ -288,7 +288,7 @@ describe('text encodings', () => {
           const written = decoder.decode(utf8Data.slice(i, i + 2), target);
           decoded += toString(target, written);
         }
-        assert(decoded, 'Ä€𝄞Ö𝄞€Ü𝄞€');
+        assert.strictEqual(decoded, 'Ä€𝄞Ö𝄞€Ü𝄞€');
       });
 
       it('2/3/4 byte sequences - advance by 3', () => {
@@ -300,7 +300,7 @@ describe('text encodings', () => {
           const written = decoder.decode(utf8Data.slice(i, i + 3), target);
           decoded += toString(target, written);
         }
-        assert(decoded, 'Ä€𝄞Ö𝄞€Ü𝄞€');
+        assert.strictEqual(decoded, 'Ä€𝄞Ö𝄞€Ü𝄞€');
       });
 
       it('BOMs (3 byte sequences) - advance by 2', () => {
@@ -312,7 +312,7 @@ describe('text encodings', () => {
           const written = decoder.decode(utf8Data.slice(i, i + 2), target);
           decoded += toString(target, written);
         }
-        assert.equal(decoded, '');
+        assert.strictEqual(decoded, '');
       });
 
       it('test break after 3 bytes - issue #2495', () => {
@@ -320,11 +320,37 @@ describe('text encodings', () => {
         const target = new Uint32Array(5);
         const utf8Data = fromByteString('\xf0\xa0\x9c\x8e');
         let written = decoder.decode(utf8Data.slice(0, 3), target);
-        assert.equal(written, 0);
+        assert.strictEqual(written, 0);
         written = decoder.decode(utf8Data.slice(3), target);
-        assert.equal(written, 1);
+        assert.strictEqual(written, 1);
-        assert(toString(target, written), '𠜎');
+        assert.strictEqual(toString(target, written), '𠜎');
       });
+
+      describe('0x80 not swallowed in continuation', () => {
+        it('A—B', () => {
+          const decoder = new Utf8ToUtf32();
+          const target = new Uint32Array(5);
+          const utf8Data = new TextEncoder().encode('A—BA—BA—BA—BA—B');
+          let decoded = '';
+          for (let i = 0; i < utf8Data.length; i += 2) {
+            const written = decoder.decode(utf8Data.slice(i, i + 2), target);
+            decoded += toString(target, written);
+          }
+          assert.strictEqual(decoded, 'A—BA—BA—BA—BA—B');
+        });
+        it('A𐀀B', () => {
+          const decoder = new Utf8ToUtf32();
+          const target = new Uint32Array(5);
+          const utf8Data = new TextEncoder().encode('A𐀀BA𐀀BA𐀀BA𐀀BA𐀀B');
+          let decoded = '';
+          for (let i = 0; i < utf8Data.length; i += 2) {
+            const written = decoder.decode(utf8Data.slice(i, i + 2), target);
+            decoded += toString(target, written);
+          }
+          assert.strictEqual(decoded, 'A𐀀BA𐀀BA𐀀BA𐀀BA𐀀B');
+        });
+      });
+
     });
   });
 });

src/common/input/TextDecoder.ts

@@ -158,9 +158,9 @@ export class Utf8ToUtf32 {
       cp &= ((((cp & 0xE0) === 0xC0)) ? 0x1F : (((cp & 0xF0) === 0xE0)) ? 0x0F : 0x07);
       let pos = 0;
       let tmp: number;
-      while ((tmp = this.interim[++pos] & 0x3F) && pos < 4) {
+      while ((tmp = this.interim[++pos]) && pos < 4) {
         cp <<= 6;
-        cp |= tmp;
+        cp |= tmp & 0x3F;
       }
       // missing bytes - read ahead from input
       const type = (((this.interim[0] & 0xE0) === 0xC0)) ? 2 : (((this.interim[0] & 0xF0) === 0xE0)) ? 3 : 4;

src/headless/Terminal.ts

@@ -99,10 +99,6 @@ export class Terminal extends CoreTerminal {
    * Clear the entire buffer, making the prompt line the new first line.
    */
   public clear(): void {
-    if (this.buffer.ybase === 0 && this.buffer.y === 0) {
-      // Don't clear if it's already clear
-      return;
-    }
     this.buffer.clearAllMarkers();
     this.buffer.lines.set(0, this.buffer.lines.get(this.buffer.ybase + this.buffer.y)!);
     this.buffer.lines.length = 1;

test/benchmark/BufferLine.benchmark.ts

@@ -0,0 +1,113 @@
+/**
+ * Copyright (c) 2026 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import { perfContext, before, RuntimeCase } from 'xterm-benchmark';
+import { BufferLine, DEFAULT_ATTR_DATA } from 'common/buffer/BufferLine';
+import { BufferLineStringCache } from 'common/buffer/BufferLineStringCache';
+import { CellData } from 'common/buffer/CellData';
+import { BgFlags, NULL_CELL_CHAR, NULL_CELL_WIDTH, NULL_CELL_CODE, UnderlineStyle } from 'common/buffer/Constants';
+
+const ITERATIONS = 500_000;
+const stringCache = new BufferLineStringCache();
+const nullCell = CellData.fromCharData([0, NULL_CELL_CHAR, NULL_CELL_WIDTH, NULL_CELL_CODE]);
+const yCell = CellData.fromCharData([0, 'y', 1, 'y'.charCodeAt(0)]);
+
+function fillPlainY(line: BufferLine, cols: number): void {
+  for (let i = 0; i < cols; i++) {
+    line.setCellFromCodepoint(i, 'y'.charCodeAt(0), 1, DEFAULT_ATTR_DATA);
+  }
+}
+
+function setupScrollRecyclePair(cols: number): { recycled: BufferLine, blank: BufferLine } {
+  const blank = new BufferLine(stringCache, cols, nullCell, false);
+  const recycled = new BufferLine(stringCache, cols, yCell, false);
+  fillPlainY(recycled, cols);
+  return { recycled, blank };
+}
+
+/** Source line with combining chars and extended attrs spread across columns. */
+function buildSparseSourceLine(cols: number): BufferLine {
+  const line = new BufferLine(stringCache, cols, nullCell, false);
+  for (let i = 0; i < cols; i++) {
+    if (i % 10 === 0) {
+      const cell = CellData.fromCharData([0, 'a', 1, 'a'.charCodeAt(0)]);
+      cell.extended.underlineStyle = UnderlineStyle.CURLY;
+      cell.bg |= BgFlags.HAS_EXTENDED;
+      line.setCell(i, cell);
+    } else if (i % 17 === 0) {
+      line.setCell(i, CellData.fromCharData([1, 'e\u0301', 1, '\u0301'.charCodeAt(0)]));
+    } else {
+      line.setCellFromCodepoint(i, 'y'.charCodeAt(0), 1, DEFAULT_ATTR_DATA);
+    }
+  }
+  return line;
+}
+
+function setupCopyFromSparsePair(cols: number): { dest: BufferLine, src: BufferLine } {
+  const src = buildSparseSourceLine(cols);
+  const dest = new BufferLine(stringCache, cols, yCell, false);
+  fillPlainY(dest, cols);
+  return { dest, src };
+}
+
+perfContext('BufferLine.copyFrom (scroll recycle)', () => {
+  perfContext('cols=80', () => {
+    let recycled: BufferLine;
+    let blank: BufferLine;
+    before(() => {
+      ({ recycled, blank } = setupScrollRecyclePair(80));
+    });
+    new RuntimeCase('', () => {
+      for (let i = 0; i < ITERATIONS; i++) {
+        recycled.copyFrom(blank);
+      }
+      return { payloadSize: ITERATIONS };
+    }, { fork: false }).showAverageRuntime();
+  });
+
+  perfContext('cols=279', () => {
+    let recycled: BufferLine;
+    let blank: BufferLine;
+    before(() => {
+      ({ recycled, blank } = setupScrollRecyclePair(279));
+    });
+    new RuntimeCase('', () => {
+      for (let i = 0; i < ITERATIONS; i++) {
+        recycled.copyFrom(blank);
+      }
+      return { payloadSize: ITERATIONS };
+    }, { fork: false }).showAverageRuntime();
+  });
+});
+
+perfContext('BufferLine.copyFrom (sparse source: combined + extended attrs)', () => {
+  perfContext('cols=80', () => {
+    let dest: BufferLine;
+    let src: BufferLine;
+    before(() => {
+      ({ dest, src } = setupCopyFromSparsePair(80));
+    });
+    new RuntimeCase('', () => {
+      for (let i = 0; i < ITERATIONS; i++) {
+        dest.copyFrom(src);
+      }
+      return { payloadSize: ITERATIONS };
+    }, { fork: false }).showAverageRuntime();
+  });
+
+  perfContext('cols=279', () => {
+    let dest: BufferLine;
+    let src: BufferLine;
+    before(() => {
+      ({ dest, src } = setupCopyFromSparsePair(279));
+    });
+    new RuntimeCase('', () => {
+      for (let i = 0; i < ITERATIONS; i++) {
+        dest.copyFrom(src);
+      }
+      return { payloadSize: ITERATIONS };
+    }, { fork: false }).showAverageRuntime();
+  });
+});