fix(BufferLine): limit copyCellsFrom metadata to copied range

copyCellsFrom copied Uint32 cell data only for the requested column
range but migrated every combined entry with key >= srcCol, which could
attach grapheme metadata to unrelated destination columns during
partial line copies in reflow and wraparound.

Copy combined and extended attributes only per copied cell and clear
stale entries on the destination when the source cell does not use
them.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/common/buffer/BufferLine.test.ts

@@ -495,6 +495,21 @@ describe('BufferLine', function(): void {
       assert.deepEqual(columns, [0]);
     });
   });
+  describe('copyCellsFrom', () => {
+    it('should only move combined data within the copied range', () => {
+      const src = new TestBufferLine(10, NULL_CELL_DATA, false);
+      src.setCell(1, createCellData(1, 'a', 1));
+      src.addCodepointToCell(1, '\u0301'.charCodeAt(0), 0);
+      src.setCell(5, createCellData(1, 'z', 1));
+      src.addCodepointToCell(5, '\u0301'.charCodeAt(0), 0);
+
+      const dest = new TestBufferLine(10, NULL_CELL_DATA, false);
+      dest.copyCellsFrom(src, 0, 0, 2, false);
+
+      assert.property(dest.combined, '1');
+      assert.notProperty(dest.combined, '5');
+    });
+  });
   describe('addCharToCell', () => {
     it('should set width to 1 for empty cell', () => {
       const line = new TestBufferLine(3, NULL_CELL_DATA, false);

src/common/buffer/BufferLine.ts

@@ -609,9 +609,13 @@ export class BufferLine implements IBufferLine {
     const srcStart = srcCol * Constants.CELL_INDICIES;
     if (src._data[srcStart + Cell.CONTENT] & Content.IS_COMBINED_MASK) {
       this._combined[destCol] = src._combined[srcCol];
+    } else {
+      delete this._combined[destCol];
     }
     if (src._data[srcStart + Cell.BG] & BgFlags.HAS_EXTENDED) {
       this._extendedAttrs[destCol] = src._extendedAttrs[srcCol];
+    } else {
+      delete this._extendedAttrs[destCol];
     }
   }