Add MicrotaskTimer and use it for decoration line index sync

Introduce MicrotaskTimer as a TimeoutTimer-style helper that schedules a
single runner via queueMicrotask, with set and cancel but no cancelAndSet.

Use it in DecorationLineCache to batch insert/delete line-index updates after
marker line adjustments, replacing ad-hoc queueMicrotask scheduling.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/common/Async.ts

@@ -73,6 +73,42 @@ export class TimeoutTimer implements IDisposable {
   }
 }
 
+/**
+ * Schedules a single runner on the microtask queue. Unlike {@link TimeoutTimer}, a scheduled
+ * microtask cannot be unqueued; {@link cancel} prevents the runner from executing if it has not
+ * run yet.
+ */
+export class MicrotaskTimer implements IDisposable {
+  private _isScheduled = false;
+  private _isDisposed = false;
+
+  public dispose(): void {
+    this.cancel();
+    this._isDisposed = true;
+  }
+
+  public cancel(): void {
+    this._isScheduled = false;
+  }
+
+  public set(runner: () => void): void {
+    if (this._isDisposed) {
+      throw new Error('Calling set on a disposed MicrotaskTimer');
+    }
+    if (this._isScheduled) {
+      return;
+    }
+    this._isScheduled = true;
+    queueMicrotask(() => {
+      if (!this._isScheduled) {
+        return;
+      }
+      this._isScheduled = false;
+      runner();
+    });
+  }
+}
+
 export class IntervalTimer implements IDisposable {
   private _disposable: IDisposable | undefined;
   private _isDisposed = false;

src/common/services/DecorationService.ts

@@ -4,6 +4,7 @@
  */
 
 import type { IDeleteEvent, IInsertEvent } from 'common/CircularList';
+import { MicrotaskTimer } from 'common/Async';
 import { css } from 'common/Color';
 import { Disposable, DisposableStore, MutableDisposable, toDisposable } from 'common/Lifecycle';
 import { IBufferService, IDecorationService, IInternalDecoration, ILogService } from 'common/services/Services';
@@ -124,10 +125,12 @@ export class DecorationLineCache extends Disposable {
   private readonly _decorationsByLine: Map<number, IInternalDecoration[]> = new Map();
   private readonly _decorations = new Set<IInternalDecoration>();
   private readonly _bufferLineListeners = this._register(new MutableDisposable<DisposableStore>());
-
+  private readonly _lineIndexSyncTimer = this._register(new MicrotaskTimer());
   private _lineIndexSyncCallbacks: (() => void)[] = [];
 
   public clear(): void {
+    this._lineIndexSyncCallbacks.length = 0;
+    this._lineIndexSyncTimer.cancel();
     this._decorationsByLine.clear();
     this._decorations.clear();
   }
@@ -203,10 +206,7 @@ export class DecorationLineCache extends Disposable {
   /** Re-index after marker line updates (buffer listeners may run before markers). */
   private _scheduleLineIndexSync(callback: () => void): void {
     this._lineIndexSyncCallbacks.push(callback);
-    if (this._lineIndexSyncCallbacks.length > 1) {
-      return;
-    }
-    queueMicrotask(() => {
+    this._lineIndexSyncTimer.set(() => {
       const callbacks = this._lineIndexSyncCallbacks;
       this._lineIndexSyncCallbacks = [];
       for (const cb of callbacks) {