WinForms/WPF/OffScreen - Use custom FocusHandler when disposing to block taking focus

This was added as a workaround for Issue #3715 which is specific to WinForms
and will hopefully be fixed upstream.

I've added the logic to all three implementations as I think the behaviour should be
consistent across all implementations.

Author: amaitland

CefSharp.OffScreen/ChromiumWebBrowser.cs

@@ -286,7 +286,11 @@ protected virtual void Dispose(bool disposing)
 
                 // Release reference to handlers, except LifeSpanHandler which is done after Disposing
                 // ManagedCefBrowserAdapter otherwise the ILifeSpanHandler.DoClose will not be invoked.
-                FreeHandlersExceptLifeSpan();
+                // We also leave FocusHandler and override with a NoFocusHandler implementation as
+                // it so we can block taking Focus (we're dispoing afterall). Issue #3715
+                FreeHandlersExceptLifeSpanAndFocus();
+
+                FocusHandler = new NoFocusHandler();
 
                 browser = null;
 

CefSharp.WinForms/ChromiumWebBrowser.cs

@@ -417,7 +417,11 @@ private void InternalDispose(bool disposing)
 
                 // Release reference to handlers, except LifeSpanHandler which is done after Disposing
                 // ManagedCefBrowserAdapter otherwise the ILifeSpanHandler.DoClose will not be invoked.
-                FreeHandlersExceptLifeSpan();
+                // We also leave FocusHandler and override with a NoFocusHandler implementation as
+                // it so we can block taking Focus (we're dispoing afterall). Issue #3715
+                FreeHandlersExceptLifeSpanAndFocus();
+
+                FocusHandler = new NoFocusHandler();
 
                 browser = null;
 

CefSharp.Wpf/ChromiumWebBrowser.cs

@@ -667,7 +667,11 @@ private void InternalDispose(bool disposing)
 
                 // Release reference to handlers, except LifeSpanHandler which is done after Disposing
                 // ManagedCefBrowserAdapter otherwise the ILifeSpanHandler.DoClose will not be invoked.
-                this.FreeHandlersExceptLifeSpan();
+                // We also leave FocusHandler and override with a NoFocusHandler implementation as
+                // it so we can block taking Focus (we're dispoing afterall). Issue #3715
+                FreeHandlersExceptLifeSpanAndFocus();
+
+                FocusHandler = new NoFocusHandler();
 
                 browser = null;
 

CefSharp/CefSharp.csproj

@@ -127,6 +127,7 @@
     <Compile Include="Internals\FreezableBase.cs" />
     <Compile Include="Internals\IBrowserRefCounter.cs" />
     <Compile Include="Internals\MimeTypeMapping.cs" />
+    <Compile Include="Internals\NoFocusHandler.cs" />
     <Compile Include="Internals\Tasks\SyncContextTaskCompletionSource.cs" />
     <Compile Include="IRegistration.cs" />
     <Compile Include="LoadUrlAsyncResponse.cs" />

CefSharp/Internals/NoFocusHandler.cs

@@ -0,0 +1,35 @@
+// Copyright © 2021 The CefSharp Authors. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
+
+namespace CefSharp.Internals
+{
+    /// <summary>
+    /// NoFocusHandler - Used when disposing of the ChromiumWebBrowser controls
+    /// Doesn't take focus for the main browser (leaves default behaviour for popup).
+    /// OnGotFocus and OnTakeFocus are both noops.
+    /// </summary>
+    public class NoFocusHandler : IFocusHandler
+    {
+        void IFocusHandler.OnGotFocus(IWebBrowser chromiumWebBrowser, IBrowser browser)
+        {
+            //NOOP   
+        }
+
+        bool IFocusHandler.OnSetFocus(IWebBrowser chromiumWebBrowser, IBrowser browser, CefFocusSource source)
+        {
+            if(browser.IsPopup)
+            {
+                return false;
+            }
+
+            //Don't take focus
+            return true;
+        }
+
+        void IFocusHandler.OnTakeFocus(IWebBrowser chromiumWebBrowser, IBrowser browser, bool next)
+        {
+            //NOOP
+        }
+    }
+}

CefSharp/Internals/Partial/ChromiumWebBrowser.Partial.cs

@@ -304,7 +304,7 @@ public Task<LoadUrlAsyncResponse> LoadUrlAsync(string url = null, Synchronizatio
         /// Sets the handler references to null.
         /// Where required also calls Dispose().
         /// </summary>
-        private void FreeHandlersExceptLifeSpan()
+        private void FreeHandlersExceptLifeSpanAndFocus()
         {
             AudioHandler?.Dispose();
             AudioHandler = null;
@@ -318,7 +318,6 @@ private void FreeHandlersExceptLifeSpan()
             DragHandler = null;
             DownloadHandler = null;
             MenuHandler = null;
-            FocusHandler = null;
             ResourceRequestHandlerFactory = null;
             RenderProcessMessageHandler = null;
         }