optimize copyFrom and clone for blank lines

src/common/buffer/BufferLine.ts

@@ -458,7 +458,7 @@ export class BufferLine implements IBufferLine {
   }
 
   /** alter to a full copy of line  */
-  public copyFrom(line: BufferLine): void {
+  public copyFrom(line: BufferLine, blank?: boolean): void {
     this._invalidateStringCache();
     if (this.length !== line.length) {
       this._data = new Uint32Array(line._data);
@@ -467,16 +467,27 @@ export class BufferLine implements IBufferLine {
       this._data.set(line._data);
     }
     this.length = line.length;
-    this._copySparseMapsFrom(line);
+    if (blank) {
+      // a blank line may never hold combined or extended attrs,
+      // thus we can skip handling them
+      this._combined = {};
+      this._extendedAttrs = {};
+    } else {
+      this._copySparseMapsFrom(line);
+    }
     this.isWrapped = line.isWrapped;
   }
 
   /** create a new clone */
-  public clone(): IBufferLine {
+  public clone(blank?: boolean): IBufferLine {
     const newLine = new BufferLine(this._stringCache, 0, undefined, false);
     newLine._data = new Uint32Array(this._data);
     newLine.length = this.length;
-    newLine._copySparseMapsFrom(this);
+    if (!blank) {
+      // a blank line may never hold combined or extended attrs,
+      // thus we can skip handling them
+      newLine._copySparseMapsFrom(this);
+    }
     newLine.isWrapped = this.isWrapped;
     return newLine;
   }

src/common/buffer/Types.ts

@@ -131,8 +131,8 @@ export interface IBufferLine {
   resize(cols: number, fill: ICellData): boolean;
   cleanupMemory(): number;
   fill(fillCellData: ICellData, respectProtect?: boolean): void;
-  copyFrom(line: IBufferLine): void;
-  clone(): IBufferLine;
+  copyFrom(line: IBufferLine, blank?: boolean): void;
+  clone(blank?: boolean): IBufferLine;
   getTrimmedLength(): number;
   getNoBgTrimmedLength(): number;
   translateToString(trimRight?: boolean, startCol?: number, endCol?: number, outColumns?: number[]): string;

src/common/services/BufferService.ts

@@ -86,12 +86,12 @@ export class BufferService extends Disposable implements IBufferService {
       // Insert the line using the fastest method
       if (bottomRow === buffer.lines.length - 1) {
         if (willBufferBeTrimmed) {
-          buffer.lines.recycle().copyFrom(newLine);
+          buffer.lines.recycle().copyFrom(newLine, true);
         } else {
-          buffer.lines.push(newLine.clone());
+          buffer.lines.push(newLine.clone(true));
         }
       } else {
-        buffer.lines.splice(bottomRow + 1, 0, newLine.clone());
+        buffer.lines.splice(bottomRow + 1, 0, newLine.clone(true));
       }
 
       // Only adjust ybase and ydisp when the buffer is not trimmed
@@ -113,7 +113,7 @@ export class BufferService extends Disposable implements IBufferService {
       // scrollback, instead we can just shift them in-place.
       const scrollRegionHeight = bottomRow - topRow + 1 /* as it's zero-based */;
       buffer.lines.shiftElements(topRow + 1, scrollRegionHeight - 1, -1);
-      buffer.lines.set(bottomRow, newLine.clone());
+      buffer.lines.set(bottomRow, newLine.clone(true));
     }
 
     // Move the viewport to the bottom of the buffer unless the user is