Improve copyFrom for empty and sparse sparse-map paths

Use shared empty-map sentinels with copy-on-write for scroll recycling,
and an inlined _data flag scan for populated sparse maps (faster than
per-cell function calls on master). Avoid slow Object.assign map copies.

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

test/benchmark/BufferLine.benchmark.ts

@@ -4,26 +4,54 @@
  */
 
 import { perfContext, before, RuntimeCase } from 'xterm-benchmark';
-import { BufferLine } from 'common/buffer/BufferLine';
+import { BufferLine, DEFAULT_ATTR_DATA } from 'common/buffer/BufferLine';
 import { BufferLineStringCache } from 'common/buffer/BufferLineStringCache';
-import { DEFAULT_ATTR_DATA } from 'common/buffer/BufferLine';
 import { CellData } from 'common/buffer/CellData';
-import { NULL_CELL_CHAR, NULL_CELL_WIDTH, NULL_CELL_CODE } from 'common/buffer/Constants';
+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 setupScrollRecyclePair(cols: number): { recycled: BufferLine; blank: BufferLine } {
+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);
-  for (let i = 0; i < cols; i++) {
-    recycled.setCellFromCodepoint(i, 'y'.charCodeAt(0), 1, DEFAULT_ATTR_DATA);
-  }
+  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;
@@ -53,3 +81,33 @@ perfContext('BufferLine.copyFrom (scroll recycle)', () => {
     }, { 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();
+  });
+});