Ensure Process.MainWindowHandle is refreshed

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Win32.cs

@@ -243,7 +243,7 @@ public IntPtr MainWindowHandle
                     EnsureState(State.IsLocal | State.HaveId);
                     _mainWindowHandle = ProcessManager.GetMainWindowHandle(_processId);
 
-                    _haveMainWindow = true;
+                    _haveMainWindow = _mainWindowHandle != IntPtr.Zero;
                 }
                 return _mainWindowHandle;
             }

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Windows.cs

@@ -116,6 +116,7 @@ public void Kill()
         private void RefreshCore()
         {
             _signaled = false;
+            _haveMainWindow = false;
         }
 
         /// <summary>Additional logic invoked when the Process is closed.</summary>

src/libraries/System.Diagnostics.Process/tests/ProcessTests.cs

@@ -15,6 +15,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.DotNet.RemoteExecutor;
+using Microsoft.DotNet.XUnitExtensions;
 using Microsoft.Win32.SafeHandles;
 using Xunit;
 using Xunit.Sdk;
@@ -1509,6 +1510,40 @@ public void MainWindowHandle_GetNotStarted_ThrowsInvalidOperationException()
             var process = new Process();
             Assert.Throws<InvalidOperationException>(() => process.MainWindowHandle);
         }
+
+        [Fact]
+        [OuterLoop]
+        [Trait(XunitConstants.Category, XunitConstants.IgnoreForCI)] // Pops UI
+        [PlatformSpecific(TestPlatforms.Windows)]
+        public void MainWindowHandle_GetWithGui_ShouldRefresh_Windows()
+        {
+            const string ExePath = "notepad.exe";
+            Assert.True(IsProgramInstalled(ExePath));
+
+            using (Process process = Process.Start(ExePath))
+            {
+                try
+                {
+                    for (int attempt = 0; attempt < 50; ++attempt)
+                    {
+                        process.Refresh();
+                        if (process.MainWindowHandle != IntPtr.Zero)
+                        {
+                            break;
+                        }
+
+                        Thread.Sleep(100);
+                    }
+
+                    Assert.NotEqual(IntPtr.Zero, process.MainWindowHandle);
+                }
+                finally
+                {
+                    process.Kill();
+                    Assert.True(process.WaitForExit(WaitInMS));
+                }
+            }
+        }
 
         [Fact]
         public void MainWindowTitle_NoWindow_ReturnsEmpty()