Fix exception on resize during write

Fixes #5588

src/common/InputHandler.ts

@@ -521,7 +521,13 @@ export class InputHandler extends Disposable implements IInputHandler {
     const wraparoundMode = this._coreService.decPrivateModes.wraparound;
     const insertMode = this._coreService.modes.insertMode;
     const curAttr = this._curAttrData;
-    let bufferRow = this._activeBuffer.lines.get(this._activeBuffer.ybase + this._activeBuffer.y)!;
+    let bufferRow = this._activeBuffer.lines.get(this._activeBuffer.ybase + this._activeBuffer.y);
+
+    // Defensive check: bufferRow can be undefined if a resize occurred mid-write due to async
+    // scheduling gaps in WriteBuffer. See https://github.com/xtermjs/xterm.js/issues/5597
+    if (!bufferRow) {
+      return;
+    }
 
     this._dirtyRowTracker.markDirty(this._activeBuffer.y);
 
@@ -586,7 +592,10 @@ export class InputHandler extends Disposable implements IInputHandler {
             this._activeBuffer.lines.get(this._activeBuffer.ybase + this._activeBuffer.y)!.isWrapped = true;
           }
           // row changed, get it again
-          bufferRow = this._activeBuffer.lines.get(this._activeBuffer.ybase + this._activeBuffer.y)!;
+          bufferRow = this._activeBuffer.lines.get(this._activeBuffer.ybase + this._activeBuffer.y);
+          if (!bufferRow) {
+            return;
+          }
           if (oldWidth > 0 && bufferRow instanceof BufferLine) {
             // Combining character widens 1 column to 2.
             // Move old character to next line.

src/common/buffer/Buffer.ts

@@ -264,6 +264,13 @@ export class Buffer implements IBuffer {
     this._cols = newCols;
     this._rows = newRows;
 
+    // Ensure the cursor position invariant: ybase + y must be within buffer bounds
+    // This can be violated during reflow or when shrinking rows
+    if (this.lines.length > 0) {
+      const maxY = Math.max(0, this.lines.length - this.ybase - 1);
+      this.y = Math.min(this.y, maxY);
+    }
+
     this._memoryCleanupQueue.clear();
     // schedule memory cleanup only, if more than 10% of the lines are affected
     if (dirtyMemoryLines > 0.1 * this.lines.length) {

test/playwright/Terminal.test.ts

@@ -402,6 +402,16 @@ test.describe('API Integration Tests', () => {
       await pollFor(ctx.page, `window.calls`, [[10, 5], [20, 15]]);
     });
 
+    test('resize during write should not throw', async () => {
+      await openTerminal(ctx, { rows: 50, cols: 80 });
+      const largeData = 'x'.repeat(10000);
+      await ctx.proxy.write(largeData);
+      await ctx.proxy.resize(80, 10);
+      await ctx.proxy.write(largeData);
+      await ctx.proxy.resize(80, 5);
+      await ctx.proxy.write(largeData);
+    });
+
     test('onTitleChange', async () => {
       await openTerminal(ctx);
       await ctx.page.evaluate(`