preserve extra info across PeekMessage (#5838) (#6001)

Co-authored-by: NSam Bent <sambent@microsoft.com>

src/Microsoft.DotNet.Wpf/src/Shared/MS/Win32/UnsafeNativeMethodsOther.cs

@@ -187,9 +187,11 @@ internal interface IInternetSecurityManager
 
 
 #if BASE_NATIVEMETHODS
-
         [DllImport(ExternDll.User32, CharSet = CharSet.Auto)]
         internal static extern IntPtr GetMessageExtraInfo();
+
+        [DllImport(ExternDll.User32, CharSet = CharSet.Auto)]
+        internal static extern IntPtr SetMessageExtraInfo(IntPtr lParam);
 #endif
 
 #if BASE_NATIVEMETHODS

src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/Threading/Dispatcher.cs

@@ -2417,8 +2417,14 @@ internal bool CriticalRequestProcessing(bool force)
                     }
                     else if (_postedProcessingType == PROCESS_FOREGROUND)
                     {
+                        // Preserve the thread's current "extra message info"
+                        // (PeekMessage overwrites it).
+                        IntPtr extraInformation = UnsafeNativeMethods.GetMessageExtraInfo();
+
                         MSG msg = new MSG();
                         UnsafeNativeMethods.PeekMessage(ref msg, new HandleRef(this, _window.Value.Handle), _msgProcessQueue, _msgProcessQueue, NativeMethods.PM_REMOVE);
+
+                        UnsafeNativeMethods.SetMessageExtraInfo(extraInformation);
                     }
                     _postedProcessingType = PROCESS_NONE;
                 }