diff --git a/src/browser/ColorContrastCache.ts b/src/browser/ColorContrastCache.ts
index fdcd9d133199a6cd6ba9bea9606a02c03ad03b3d..0558a8873f680e8fb2a27cc833c49c5ba658e1dd 100644
--- a/src/browser/ColorContrastCache.ts
+++ b/src/browser/ColorContrastCache.ts
@@ -7,11 +7,17 @@ import { IColorContrastCache } from './Types';
 import { IColor } from '../common/Types';
 import { TwoKeyMap } from '../common/MultiKeyMap';
 
+const CONTRAST_CACHE_MAX_ENTRIES = 4096;
+
 export class ColorContrastCache implements IColorContrastCache {
   private _color: TwoKeyMap</* bg */number, /* fg */number, IColor | null> = new TwoKeyMap();
   private _css: TwoKeyMap</* bg */number, /* fg */number, string | null> = new TwoKeyMap();
+  private _entryCount = 0;
 
   public setCss(bg: number, fg: number, value: string | null): void {
+    if (this._css.get(bg, fg) === undefined) {
+      this._admitNewEntry();
+    }
     this._css.set(bg, fg, value);
   }
 
@@ -20,6 +26,9 @@ export class ColorContrastCache implements IColorContrastCache {
   }
 
   public setColor(bg: number, fg: number, value: IColor | null): void {
+    if (this._color.get(bg, fg) === undefined) {
+      this._admitNewEntry();
+    }
     this._color.set(bg, fg, value);
   }
 
@@ -30,5 +39,14 @@ export class ColorContrastCache implements IColorContrastCache {
   public clear(): void {
     this._color.clear();
     this._css.clear();
+    this._entryCount = 0;
+  }
+
+  private _admitNewEntry(): void {
+    // Color pairs outlive atlas pages, including cached misses and DOM-rendered colors.
+    if (this._entryCount >= CONTRAST_CACHE_MAX_ENTRIES) {
+      this.clear();
+    }
+    this._entryCount++;
   }
 }
diff --git a/src/common/buffer/Buffer.ts b/src/common/buffer/Buffer.ts
index e3a88bfeb4bbf79214d799adec30cba8e4dc1b49..f11e55306dde553d8f68b17340c9022cd749e914 100644
--- a/src/common/buffer/Buffer.ts
+++ b/src/common/buffer/Buffer.ts
@@ -504,7 +504,7 @@ export class Buffer extends Disposable implements IBuffer {
       for (let i = Math.min(this.lines.maxLength - 1, originalLinesLength + countToInsert - 1); i >= 0; i--) {
         if (nextToInsert && nextToInsert.start > originalLineIndex + countInsertedSoFar) {
           // Insert extra lines here, adjusting i as needed
-          for (let nextI = nextToInsert.newLines.length - 1; nextI >= 0; nextI--) {
+          for (let nextI = nextToInsert.newLines.length - 1; nextI >= 0 && i >= 0; nextI--) {
             this.lines.set(i--, nextToInsert.newLines[nextI]);
           }
           i++;
diff --git a/src/common/buffer/BufferLine.ts b/src/common/buffer/BufferLine.ts
index 4984f3a49abe8437475c206db1345f5f5fe3979c..43fd5c5b286f23f11d407c9098062e970b6a2ec6 100644
--- a/src/common/buffer/BufferLine.ts
+++ b/src/common/buffer/BufferLine.ts
@@ -116,12 +116,16 @@ export class BufferLine implements IBufferLine {
    * @deprecated
    */
   public set(index: number, value: CharData): void {
+    this._cache = '';
     this._cacheValid = false;
     this._data[index * Constants.CELL_INDICIES + Cell.FG] = value[CHAR_DATA_ATTR_INDEX];
     if (value[CHAR_DATA_CHAR_INDEX].length > 1) {
       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 {
+      if ((this._data[index * Constants.CELL_INDICIES + Cell.CONTENT] & Content.IS_COMBINED_MASK)) {
+        delete this._combined[index];
+      }
       this._data[index * Constants.CELL_INDICIES + Cell.CONTENT] = value[CHAR_DATA_CHAR_INDEX].charCodeAt(0) | (value[CHAR_DATA_WIDTH_INDEX] << Content.WIDTH_SHIFT);
     }
   }
@@ -226,12 +230,21 @@ export class BufferLine implements IBufferLine {
    * Set data at `index` to `cell`.
    */
   public setCell(index: number, cell: ICellData): void {
+    this._cache = '';
     this._cacheValid = false;
     if (cell.content & Content.IS_COMBINED_MASK) {
       this._combined[index] = cell.combinedData;
+    } else {
+      if ((this._data[index * Constants.CELL_INDICIES + Cell.CONTENT] & Content.IS_COMBINED_MASK)) {
+        delete this._combined[index];
+      }
     }
     if (cell.bg & BgFlags.HAS_EXTENDED) {
       this._extendedAttrs[index] = cell.extended;
+    } else {
+      if ((this._data[index * Constants.CELL_INDICIES + Cell.BG] & BgFlags.HAS_EXTENDED)) {
+        delete this._extendedAttrs[index];
+      }
     }
     this._data[index * Constants.CELL_INDICIES + Cell.CONTENT] = cell.content;
     this._data[index * Constants.CELL_INDICIES + Cell.FG] = cell.fg;
@@ -244,9 +257,17 @@ export class BufferLine implements IBufferLine {
    * it gets an optimized access method.
    */
   public setCellFromCodepoint(index: number, codePoint: number, width: number, attrs: IAttributeData): void {
+    this._cache = '';
     this._cacheValid = false;
+    if ((this._data[index * Constants.CELL_INDICIES + Cell.CONTENT] & Content.IS_COMBINED_MASK)) {
+      delete this._combined[index];
+    }
     if (attrs.bg & BgFlags.HAS_EXTENDED) {
       this._extendedAttrs[index] = attrs.extended;
+    } else {
+      if ((this._data[index * Constants.CELL_INDICIES + Cell.BG] & BgFlags.HAS_EXTENDED)) {
+        delete this._extendedAttrs[index];
+      }
     }
     const $idx = index * Constants.CELL_INDICIES;
     this._data[$idx + Cell.CONTENT] = codePoint | (width << Content.WIDTH_SHIFT);
@@ -261,6 +282,7 @@ export class BufferLine implements IBufferLine {
    * by the previous `setDataFromCodePoint` call, we can omit it here.
    */
   public addCodepointToCell(index: number, codePoint: number, width: number): void {
+    this._cache = '';
     this._cacheValid = false;
     let content = this._data[index * Constants.CELL_INDICIES + Cell.CONTENT];
     if (content & Content.IS_COMBINED_MASK) {
@@ -288,6 +310,7 @@ export class BufferLine implements IBufferLine {
   }
 
   public insertCells(pos: number, n: number, fillCellData: ICellData): void {
+    this._cache = '';
     this._cacheValid = false;
     pos %= this.length;
 
@@ -309,6 +332,8 @@ export class BufferLine implements IBufferLine {
       }
     }
 
+    $workCell.combinedData = '';
+
     // handle fullwidth at line end: reset last cell if it is first cell of a wide char
     if (this.getWidth(this.length - 1) === 2) {
       this.setCellFromCodepoint(this.length - 1, 0, 1, fillCellData);
@@ -316,6 +341,7 @@ export class BufferLine implements IBufferLine {
   }
 
   public deleteCells(pos: number, n: number, fillCellData: ICellData): void {
+    this._cache = '';
     this._cacheValid = false;
     pos %= this.length;
     if (n < this.length - pos) {
@@ -331,6 +357,8 @@ export class BufferLine implements IBufferLine {
       }
     }
 
+    $workCell.combinedData = '';
+
     // handle fullwidth at pos:
     // - reset pos-1 if wide char
     // - reset pos if width==0 (previous second cell of a wide char)
@@ -343,6 +371,7 @@ export class BufferLine implements IBufferLine {
   }
 
   public replaceCells(start: number, end: number, fillCellData: ICellData, respectProtect: boolean = false): void {
+    this._cache = '';
     this._cacheValid = false;
     // full branching on respectProtect==true, hopefully getting fast JIT for standard case
     if (respectProtect) {
@@ -383,6 +412,7 @@ export class BufferLine implements IBufferLine {
    * excess memory (true after shrinking > Constants.CLEANUP_THRESHOLD).
    */
   public resize(cols: number, fillCellData: ICellData): boolean {
+    this._cache = '';
     this._cacheValid = false;
     if (cols === this.length) {
       return this._data.length * 4 * Constants.CLEANUP_THRESHOLD < this._data.buffer.byteLength;
@@ -443,6 +473,7 @@ export class BufferLine implements IBufferLine {
 
   /** fill a line with fillCharData */
   public fill(fillCellData: ICellData, respectProtect: boolean = false): void {
+    this._cache = '';
     this._cacheValid = false;
     // full branching on respectProtect==true, hopefully getting fast JIT for standard case
     if (respectProtect) {
@@ -515,6 +546,7 @@ export class BufferLine implements IBufferLine {
   }
 
   public copyCellsFrom(src: BufferLine, srcCol: number, destCol: number, length: number, applyInReverse: boolean): void {
+    this._cache = '';
     this._cacheValid = false;
     const srcData = src._data;
     if (applyInReverse) {
@@ -596,9 +628,17 @@ 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 {
+      if (this._combined[destCol] !== undefined) {
+        delete this._combined[destCol];
+      }
     }
     if (src._data[srcStart + Cell.BG] & BgFlags.HAS_EXTENDED) {
       this._extendedAttrs[destCol] = src._extendedAttrs[srcCol];
+    } else {
+      if (this._extendedAttrs[destCol] !== undefined) {
+        delete this._extendedAttrs[destCol];
+      }
     }
   }