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);
   }