diff --git a/src/common/buffer/BufferLine.ts b/src/common/buffer/BufferLine.ts
index 4b6f6e8bb..30b4ed3bb 100644
--- a/src/common/buffer/BufferLine.ts
+++ b/src/common/buffer/BufferLine.ts
@@ -73,7 +73,9 @@ export interface IBufferLineStringCache {
  */
 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} = {};
+  /** Sparse cache; only read when `HAS_EXTENDED` is set in `_data`. */
   protected _extendedAttrs: {[index: number]: IExtendedAttrs | undefined} = {};
   protected _stringCacheEntryRef: WeakRef<IBufferLineStringCacheEntry> | undefined;
   public length: number;
@@ -205,9 +207,15 @@ export class BufferLine implements IBufferLine {
     cell.bg = this._data[$startIndex + Cell.BG];
     if (cell.content & Content.IS_COMBINED_MASK) {
       cell.combinedData = this._combined[index];
+    } else {
+      cell.combinedData = '';
     }
     if (cell.bg & BgFlags.HAS_EXTENDED) {
       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).
+      cell.extended = DEFAULT_ATTR_DATA.extended.clone();
     }
     return cell;
   }
@@ -459,14 +467,7 @@ export class BufferLine implements IBufferLine {
       this._data.set(line._data);
     }
     this.length = line.length;
-    this._combined = {};
-    for (const el in line._combined) {
-      this._combined[el] = line._combined[el];
-    }
-    this._extendedAttrs = {};
-    for (const el in line._extendedAttrs) {
-      this._extendedAttrs[el] = line._extendedAttrs[el];
-    }
+    this._copySparseMapsFrom(line);
     this.isWrapped = line.isWrapped;
   }
 
@@ -475,12 +476,7 @@ export class BufferLine implements IBufferLine {
     const newLine = new BufferLine(this._stringCache, 0, undefined, false);
     newLine._data = new Uint32Array(this._data);
     newLine.length = this.length;
-    for (const el in this._combined) {
-      newLine._combined[el] = this._combined[el];
-    }
-    for (const el in this._extendedAttrs) {
-      newLine._extendedAttrs[el] = this._extendedAttrs[el];
-    }
+    newLine._copySparseMapsFrom(this);
     newLine.isWrapped = this.isWrapped;
     return newLine;
   }
@@ -511,27 +507,14 @@ export class BufferLine implements IBufferLine {
         for (let i = 0; i < Constants.CELL_INDICIES; i++) {
           this._data[(destCol + cell) * Constants.CELL_INDICIES + i] = srcData[(srcCol + cell) * Constants.CELL_INDICIES + i];
         }
-        if (srcData[(srcCol + cell) * Constants.CELL_INDICIES + Cell.BG] & BgFlags.HAS_EXTENDED) {
-          this._extendedAttrs[destCol + cell] = src._extendedAttrs[srcCol + cell];
-        }
+        this._copyCellMapsFrom(src, srcCol + cell, destCol + cell);
       }
     } else {
       for (let cell = 0; cell < length; cell++) {
         for (let i = 0; i < Constants.CELL_INDICIES; i++) {
           this._data[(destCol + cell) * Constants.CELL_INDICIES + i] = srcData[(srcCol + cell) * Constants.CELL_INDICIES + i];
         }
-        if (srcData[(srcCol + cell) * Constants.CELL_INDICIES + Cell.BG] & BgFlags.HAS_EXTENDED) {
-          this._extendedAttrs[destCol + cell] = src._extendedAttrs[srcCol + cell];
-        }
-      }
-    }
-
-    // Move any combined data over as needed, FIXME: repeat for extended attrs
-    const srcCombinedKeys = Object.keys(src._combined);
-    for (let i = 0; i < srcCombinedKeys.length; i++) {
-      const key = parseInt(srcCombinedKeys[i], 10);
-      if (key >= srcCol) {
-        this._combined[key - srcCol + destCol] = src._combined[key];
+        this._copyCellMapsFrom(src, srcCol + cell, destCol + cell);
       }
     }
   }
@@ -620,4 +603,24 @@ export class BufferLine implements IBufferLine {
       cacheEntry.isTrimmed = false;
     }
   }
+
+  /** Copy sparse map entries for a single cell when `_data` flags require them. */
+  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._combined[destCol] = src._combined[srcCol];
+    }
+    if (src._data[srcStart + Cell.BG] & BgFlags.HAS_EXTENDED) {
+      this._extendedAttrs[destCol] = src._extendedAttrs[srcCol];
+    }
+  }
+
+  /** Rebuild sparse maps from another line, keyed only by `_data` flags. */
+  private _copySparseMapsFrom(line: BufferLine): void {
+    this._combined = {};
+    this._extendedAttrs = {};
+    for (let i = 0; i < line.length; i++) {
+      this._copyCellMapsFrom(line, i, i);
+    }
+  }
 }