From 598c0ec62aaf06b5e78a994f44113c6dfef527cf Mon Sep 17 00:00:00 2001
From: ET <evgenyt@microsoft.com>
Date: Fri, 27 Feb 2026 10:47:47 -0800
Subject: [PATCH] Input/Stylus: Make WISP pen processing more resilient against
 disposal

- Add IsDisposed property to PenThreadWorker exposing the __disposed flag
- Add disposed guard to WorkerGetTabletsInfo returning Array.Empty immediately
  (consistent with all other Worker* methods that already have this guard)
- Expose IsDisposed on PenThread via forwarding property
- PenThreadPool: prune disposed PenThread entries alongside dead WeakReferences
- PenThread: make _penThreadWorker field readonly; remove now-redundant null check
---
 .../Windows/Input/Stylus/Wisp/PenThread.cs      |  7 ++++---
 .../Windows/Input/Stylus/Wisp/PenThreadPool.cs  | 17 ++++++-----------
 .../Input/Stylus/Wisp/PenThreadWorker.cs        |  7 +++++++
 3 files changed, 17 insertions(+), 14 deletions(-)

diff --git a/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThread.cs b/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThread.cs
index a46727e0e11..8a34f405dd8 100644
--- a/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThread.cs
+++ b/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);
diff --git a/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThreadPool.cs b/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThreadPool.cs
index b2673c26e8a..fd42deca801 100644
--- a/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThreadPool.cs
+++ b/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;
                     }
                 }
 
diff --git a/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThreadWorker.cs b/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThreadWorker.cs
index a873eeb5860..b0787c4d8b8 100644
--- a/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/PenThreadWorker.cs
+++ b/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();