Handle empty queue in WaitForPendingWorkAsync

Cancellation allows item removal from the pending work queue from non-UI threads,
so the UI thread handling needs to account for the possibility of it being empty
after WaitForItemsAsync completes.

src/EditorFeatures/Core/Implementation/ForegroundNotification/ForegroundNotificationService.cs

@@ -196,26 +196,29 @@ private void NotifyOnForegroundWorker()
 
         private async Task WaitForPendingWorkAsync()
         {
-            await _workQueue.WaitForItemsAsync().ConfigureAwait(false);
             while (true)
             {
-                var current = Environment.TickCount;
-                var nextItem = _workQueue.PeekNextItemTime();
+                await _workQueue.WaitForItemsAsync().ConfigureAwait(false);
+                if (!_workQueue.TryPeekNextItemTime(out var nextItem))
+                {
+                    // Need to go back and wait for an item
+                    continue;
+                }
 
                 // The next item is ready to run
-                if (nextItem - current <= 0)
+                if (nextItem - Environment.TickCount <= 0)
                 {
                     break;
                 }
 
                 // wait some and re-check since there could be another one inserted before the first one while we were waiting.
-                await Task.Delay(MinimumDelayBetweenProcessing).ConfigureAwait(continueOnCapturedContext: false);
+                await Task.Delay(MinimumDelayBetweenProcessing).ConfigureAwait(false);
             }
 
             // Throttle how often we run by waiting MinimumDelayBetweenProcessing since the last time we processed notifications
             if (Environment.TickCount - _lastProcessedTimeInMS < MinimumDelayBetweenProcessing)
             {
-                await Task.Delay(MinimumDelayBetweenProcessing).ConfigureAwait(continueOnCapturedContext: false);
+                await Task.Delay(MinimumDelayBetweenProcessing).ConfigureAwait(false);
             }
         }
 
@@ -344,12 +347,18 @@ private void Enqueue_NoLock(LinkedListNode<PendingWork> entry)
                 return;
             }
 
-            public int PeekNextItemTime()
+            public bool TryPeekNextItemTime(out int minimumRunPoint)
             {
                 lock (_gate)
                 {
-                    Contract.Requires(_list.Count > 0);
-                    return _list.First.Value.MinimumRunPointInMS;
+                    if (_list.Count == 0)
+                    {
+                        minimumRunPoint = 0;
+                        return false;
+                    }
+
+                    minimumRunPoint = _list.First.Value.MinimumRunPointInMS;
+                    return true;
                 }
             }