Input/Stylus: Make WISP pen processing more resilient against disposal

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThread.cs

@@ -11,7 +11,7 @@ namespace System.Windows.Input
 
     internal sealed class PenThread
     {
-        private PenThreadWorker _penThreadWorker;
+        private readonly PenThreadWorker _penThreadWorker;
 
         internal PenThread()
         {
@@ -37,13 +37,14 @@ internal void Dispose()
 
         private void DisposeHelper()
         {
-            // NOTE: PenThreadWorker deals with already being disposed logic.
-            _penThreadWorker?.Dispose();
+            _penThreadWorker.Dispose();
             GC.KeepAlive(this);
         }
 
         /////////////////////////////////////////////////////////////////////
 
+        internal bool IsDisposed => _penThreadWorker.IsDisposed;
+
         internal bool AddPenContext(PenContext penContext)
         {
             return _penThreadWorker.WorkerAddPenContext(penContext);

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThreadPool.cs

@@ -93,20 +93,15 @@ private PenThread GetPenThreadForPenContextHelper(PenContext penContext)
                 // We scan back to front to enable list cleanup.
                 for (int i = _penThreadWeakRefList.Count - 1; i >= 0; i--)
                 {
-                    PenThread candidatePenThread = null;
-
-                    // Select a thread if it's a valid WeakReference and we're not ignoring it
-                    // Allow selection to happen multiple times so we get the first valid candidate
-                    // in forward order.
-                    if (_penThreadWeakRefList[i].TryGetTarget(out candidatePenThread)
-                        && !ignoredThreads.Contains(candidatePenThread))
+                    // Check if the WeakReference is still alive and not disposed.
+                    if (!_penThreadWeakRefList[i].TryGetTarget(out PenThread candidatePenThread) ||
+                        candidatePenThread.IsDisposed)
                     {
-                        selectedPenThread = candidatePenThread;
+                        _penThreadWeakRefList.RemoveAt(i);
                     }
-                    // This is an invalid WeakReference and should be removed
-                    else if (candidatePenThread == null)
+                    else if (!ignoredThreads.Contains(candidatePenThread))
                     {
-                        _penThreadWeakRefList.RemoveAt(i);
+                        selectedPenThread = candidatePenThread;
                     }
                 }
 

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThreadWorker.cs

@@ -559,8 +559,15 @@ internal bool WorkerRemovePenContext(PenContext penContext)
 
         /////////////////////////////////////////////////////////////////////
 
+        internal bool IsDisposed => __disposed;
+
         internal TabletDeviceInfo[] WorkerGetTabletsInfo()
         {
+            if (__disposed)
+            {
+                return Array.Empty<TabletDeviceInfo>();
+            }
+
             // Set data up for this call
             WorkerOperationGetTabletsInfo getTablets = new WorkerOperationGetTabletsInfo();