Fix flushSync() to fire onWriteParsed

flushSync() processes pending writes synchronously (e.g. during
CoreTerminal.resize()) but did not emit onWriteParsed, unlike the
async _innerWrite path. Addons listening for parse completion could
miss updates after resize.

Fixes #5912

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

src/common/input/WriteBuffer.test.ts

@@ -125,5 +125,20 @@ describe('WriteBuffer', () => {
       assert.deepEqual(stack, []);
       assert.deepEqual(cbStack, []);
     });
+    it('flushSync fires onWriteParsed', () => {
+      let parsed = 0;
+      wb.onWriteParsed(() => parsed++);
+      wb.write('a');
+      wb.write('b');
+      assert.equal(parsed, 0);
+      wb.flushSync();
+      assert.equal(parsed, 1);
+    });
+    it('flushSync with no pending writes does not fire onWriteParsed', () => {
+      let parsed = 0;
+      wb.onWriteParsed(() => parsed++);
+      wb.flushSync();
+      assert.equal(parsed, 0);
+    });
   });
 });

src/common/input/WriteBuffer.ts

@@ -71,7 +71,9 @@ export class WriteBuffer extends Disposable {
 
     // Process all pending chunks synchronously
     let chunk: string | Uint8Array | undefined;
+    let didProcess = false;
     while (chunk = this._writeBuffer.shift()) {
+      didProcess = true;
       this._action(chunk);
       const cb = this._callbacks.shift();
       if (cb) cb();
@@ -84,6 +86,9 @@ export class WriteBuffer extends Disposable {
     this._callbacks.length = 0;
 
     this._isSyncWriting = false;
+    if (didProcess) {
+      this._onWriteParsed.fire();
+    }
   }
 
   /**