Add WriteBuffer regression tests for dispose and hot paths

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

src/common/input/WriteBuffer.test.ts

@@ -160,14 +160,51 @@ describe('WriteBuffer', () => {
     });
     it('write after dispose is a no-op', done => {
       wb.dispose();
-      wb.write('a', () => {
-        assert.deepEqual(stack, []);
-        done();
-      });
+      wb.write('a');
       setTimeout(() => {
         assert.deepEqual(stack, []);
         done();
       }, 20);
     });
+    it('dispose is idempotent', done => {
+      wb.write('a');
+      wb.dispose();
+      wb.dispose();
+      setTimeout(() => {
+        assert.deepEqual(stack, []);
+        done();
+      }, 20);
+    });
+    it('async handler continuation is skipped after dispose', done => {
+      let resolve!: (value: boolean) => void;
+      const pending = new Promise<boolean>(r => { resolve = r; });
+      wb = new WriteBuffer(() => pending);
+      wb.write('a');
+      wb.dispose();
+      resolve(true);
+      setTimeout(() => {
+        assert.deepEqual(stack, []);
+        done();
+      }, 20);
+    });
+    it('handleUserInput still processes first chunk synchronously', () => {
+      wb.handleUserInput();
+      wb.write('a');
+      assert.deepEqual(stack, ['a']);
+    });
+    it('flushSync after dispose is a no-op', done => {
+      wb.write('a');
+      wb.dispose();
+      wb.flushSync();
+      setTimeout(() => {
+        assert.deepEqual(stack, []);
+        done();
+      }, 20);
+    });
+    it('writeSync after dispose is a no-op', () => {
+      wb.dispose();
+      wb.writeSync('a');
+      assert.deepEqual(stack, []);
+    });
   });
 });