Flush writes on resize

Fixes #5597

src/common/CoreTerminal.ts

@@ -176,6 +176,10 @@ export abstract class CoreTerminal extends Disposable implements ICoreTerminal {
     x = Math.max(x, MINIMUM_COLS);
     y = Math.max(y, MINIMUM_ROWS);
 
+    // Flush pending writes before resize to avoid race conditions where async
+    // writes are processed with incorrect dimensions
+    this._writeBuffer.flushSync();
+
     this._bufferService.resize(x, y);
   }
 

src/common/input/WriteBuffer.test.ts

@@ -106,5 +106,24 @@ describe('WriteBuffer', () => {
       wb.writeSync('1', 10);
       assert.equal(last, '11'); // 1 + 10 sub calls = 11
     });
+    it('flushSync processes all pending writes', done => {
+      wb.write('a', () => { cbStack.push('a'); });
+      wb.write('b', () => { cbStack.push('b'); });
+      wb.write('c', () => { cbStack.push('c'); });
+      wb.flushSync();
+      assert.deepEqual(stack, ['a', 'b', 'c']);
+      assert.deepEqual(cbStack, ['a', 'b', 'c']);
+      wb.write('x', () => { cbStack.push('x'); });
+      wb.write('', () => {
+        assert.deepEqual(stack, ['a', 'b', 'c', 'x', '']);
+        assert.deepEqual(cbStack, ['a', 'b', 'c', 'x']);
+        done();
+      });
+    });
+    it('flushSync with no pending writes is a no-op', () => {
+      wb.flushSync();
+      assert.deepEqual(stack, []);
+      assert.deepEqual(cbStack, []);
+    });
   });
 });

src/common/input/WriteBuffer.ts

@@ -54,6 +54,38 @@ export class WriteBuffer extends Disposable {
     this._didUserInput = true;
   }
 
+  /**
+   * Flushes all pending writes synchronously. This is useful when you need to
+   * ensure all queued data is processed before performing an operation that
+   * depends upon everything being parsed like resize.
+   *
+   * Note: This is unreliable with async parser handlers as it does not wait for
+   * promises to resolve.
+   */
+  public flushSync(): void {
+    // exit early if another sync write loop is active
+    if (this._isSyncWriting) {
+      return;
+    }
+    this._isSyncWriting = true;
+
+    // Process all pending chunks synchronously
+    let chunk: string | Uint8Array | undefined;
+    while (chunk = this._writeBuffer.shift()) {
+      this._action(chunk);
+      const cb = this._callbacks.shift();
+      if (cb) cb();
+    }
+
+    // Reset buffer state
+    this._pendingData = 0;
+    this._bufferOffset = 0x7FFFFFFF;
+    this._writeBuffer.length = 0;
+    this._callbacks.length = 0;
+
+    this._isSyncWriting = false;
+  }
+
   /**
    * @deprecated Unreliable, to be removed soon.
    */