Always spawn message loop thread for SystemEvents

src/libraries/Microsoft.Win32.SystemEvents/src/Microsoft/Win32/SystemEvents.cs

@@ -469,33 +469,23 @@ private static void EnsureSystemEvents(bool requireHandle)
                 {
                     if (s_systemEvents == null)
                     {
-                        // If we are creating system events on a thread declared as STA, then
-                        // just share the thread.
-                        if (!UserInteractive || Thread.CurrentThread.GetApartmentState() == ApartmentState.STA)
-                        {
-                            SystemEvents systemEvents = new SystemEvents();
-                            systemEvents.Initialize();
+                        // Create a new pumping thread.  We always create one even if the current thread
+                        // is STA, as there are no guarantees this thread will pump nor still be alive
+                        // for the desired duration.
 
-                            // ensure this is initialized last as that will force concurrent threads calling
-                            // this method to block until after we've initialized.
-                            s_systemEvents = systemEvents;
-                        }
-                        else
+                        s_eventWindowReady = new ManualResetEvent(false);
+                        SystemEvents systemEvents = new SystemEvents();
+                        s_windowThread = new Thread(new ThreadStart(systemEvents.WindowThreadProc))
                         {
-                            s_eventWindowReady = new ManualResetEvent(false);
-                            SystemEvents systemEvents = new SystemEvents();
-                            s_windowThread = new Thread(new ThreadStart(systemEvents.WindowThreadProc))
-                            {
-                                IsBackground = true,
-                                Name = ".NET System Events"
-                            };
-                            s_windowThread.Start();
-                            s_eventWindowReady.WaitOne();
-
-                            // ensure this is initialized last as that will force concurrent threads calling
-                            // this method to block until after we've initialized.
-                            s_systemEvents = systemEvents;
-                        }
+                            IsBackground = true,
+                            Name = ".NET System Events"
+                        };
+                        s_windowThread.Start();
+                        s_eventWindowReady.WaitOne();
+
+                        // ensure this is initialized last as that will force concurrent threads calling
+                        // this method to block until after we've initialized.
+                        s_systemEvents = systemEvents;
 
                         if (requireHandle && s_systemEvents._windowHandle == IntPtr.Zero)
                         {

src/libraries/Microsoft.Win32.SystemEvents/tests/Microsoft.Win32.SystemEvents.Tests.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent)-windows;net461-windows</TargetFrameworks>
+    <IncludeRemoteExecutor>true</IncludeRemoteExecutor>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(CommonPath)Interop\Windows\Interop.Libraries.cs"

src/libraries/Microsoft.Win32.SystemEvents/tests/SystemEvents.InvokeOnEventsThread.cs

@@ -2,10 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Runtime.InteropServices;
 using System.Threading;
+using Microsoft.DotNet.RemoteExecutor;
 using Xunit;
 using static Interop;
 
@@ -53,5 +51,31 @@ public void InvokeOnEventsThreadRunsOnSameThreadAsOtherEvents()
                 invoked.Dispose();
             }
         }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWindowsNanoNorServerCore))]
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework)]
+        public void RegisterFromSTAThreadThatGoesAway_MessageStillDelivered()
+        {
+            RemoteExecutor.Invoke(() => // to ensure no one has registered for any events before
+            {
+                bool changing = false, changed = false;
+
+                // Register for the events on an STA thread that then immediately exits
+                var thread = new Thread(() =>
+                {
+                    SystemEvents.DisplaySettingsChanging += (o, e) => changing = true;
+                    SystemEvents.DisplaySettingsChanged += (o, e) => changed = true;
+                });
+                thread.SetApartmentState(ApartmentState.STA);
+                thread.IsBackground = true;
+                thread.Start();
+                thread.Join();
+
+                SendMessage(User32.WM_REFLECT + User32.WM_DISPLAYCHANGE, IntPtr.Zero, IntPtr.Zero);
+
+                Assert.True(changing);
+                Assert.True(changed);
+            });
+        }
     }
 }