WinForms - Demo workaround for activation issue when showing two browser forms

When there are two forms that contain browser instances shown at the same time they rapidly switch between
the two windows.

Issue #2928

Author: amaitland

CefSharp.WinForms.Example/CefSharp.WinForms.Example.csproj

@@ -89,6 +89,7 @@
     <Compile Include="Handlers\KeyboardHandler.cs" />
     <Compile Include="Handlers\LifeSpanHandler.cs" />
     <Compile Include="Handlers\MenuHandler.cs" />
+    <Compile Include="Handlers\MultiFormFocusHandler.cs" />
     <Compile Include="Handlers\ScheduleMessagePumpBrowserProcessHandler.cs" />
     <Compile Include="Handlers\WinFormsBrowserProcessHandler.cs" />
     <Compile Include="Handlers\WinFormsRequestHandler.cs" />

CefSharp.WinForms.Example/Handlers/MultiFormFocusHandler.cs

@@ -0,0 +1,126 @@
+// Copyright © 2019 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.
+
+using CefSharp.WinForms.Internals;
+
+namespace CefSharp.WinForms.Example.Handlers
+{
+    /// <summary>
+    /// Implementation of <see cref="IFocusHandler"/> that shows a workaround
+    /// for https://github.com/cefsharp/CefSharp/issues/2928
+    /// This version is based on the old behaviour seen in version 71 etc
+    /// </summary>
+    /// <seealso cref="CefSharp.IFocusHandler" />
+    public class MultiFormFocusHandler : IFocusHandler
+    {
+        /// <summary>
+        /// Called when the browser component has received focus.
+        /// </summary>
+        /// <param name="chromiumWebBrowser">the ChromiumWebBrowser control</param>
+        /// <param name="browser">the browser object</param>
+        /// <remarks>Try to avoid needing to override this logic in a subclass. The implementation in
+        /// DefaultFocusHandler relies on very detailed behavior of how WinForms and
+        /// Windows interact during window activation.</remarks>
+        void IFocusHandler.OnGotFocus(IWebBrowser chromiumWebBrowser, IBrowser browser)
+        {
+            //We don't deal with popups as they're rendered by default entirely by CEF
+            //For print dialogs the browser will be null, we don't want to deal with that either.
+            if (browser == null || browser.IsPopup)
+            {
+                return;
+            }
+
+            var winFormsChromiumWebBrowser = (ChromiumWebBrowser)chromiumWebBrowser;
+            // During application activation, CEF receives a WM_SETFOCUS
+            // message from Windows because it is the top window 
+            // on the CEF UI thread.
+            //
+            // If the WinForm ChromiumWebBrowser control is the 
+            // current .ActiveControl before app activation 
+            // then we MUST NOT try to reactivate the WinForm
+            // control during activation because that will 
+            // start a race condition between reactivating
+            // the CEF control AND having another control 
+            // that should be the new .ActiveControl.
+            //
+            // For example:
+            // * CEF control has focus, and thus ChromiumWebBrowser
+            //   is the current .ActiveControl
+            // * Alt-Tab to another application
+            // * Click a non CEF control in the WinForms application.
+            // * This begins the Windows activation process.
+            // * The WM_ACTIVATE process on the WinForm UI thread
+            //   will update .ActiveControl to the clicked control.
+            //   The clicked control will receive WM_SETFOCUS as well. 
+            //   (i.e. OnGotFocus)
+            //   If the ChromiumWebBrowser was the previous .ActiveControl,
+            //   then we set .Activating = true.
+            // * The WM_ACTIVATE process on the CEF thread will
+            //   send WM_SETFOCUS to CEF thus staring the race of
+            //   which will end first, the WndProc WM_ACTIVATE process
+            //   on the WinForm UI thread or the WM_ACTIVATE process
+            //   on the CEF UI thread.
+            // * CEF will then call this method on the CEF UI thread
+            //   due to WM_SETFOCUS.
+            // * This method will clear the activation state (if any)
+            //   on the ChromiumWebBrowser control, due to the race
+            //   condition the WinForm UI thread cannot.
+            if (winFormsChromiumWebBrowser.IsActivating)
+            {
+                winFormsChromiumWebBrowser.IsActivating = false;
+            }
+            else
+            {
+                // Otherwise, we're not being activated
+                // so we must activate the ChromiumWebBrowser control
+                // for WinForms focus tracking.
+                winFormsChromiumWebBrowser.InvokeOnUiThreadIfRequired(() =>
+                {
+                    winFormsChromiumWebBrowser.Activate();
+                });
+            }
+        }
+
+        /// <summary>
+        /// Called when the browser component is requesting focus.
+        /// </summary>
+        /// <param name="chromiumWebBrowser">the ChromiumWebBrowser control</param>
+        /// <param name="browser">the browser object</param>
+        /// <param name="source">Indicates where the focus request is originating from.</param>
+        /// <returns>Return false to allow the focus to be set or true to cancel setting the focus.</returns>
+        bool IFocusHandler.OnSetFocus(IWebBrowser chromiumWebBrowser, IBrowser browser, CefFocusSource source)
+        {
+            //We don't deal with popups as they're rendered by default entirely by CEF
+            if (browser.IsPopup)
+            {
+                return false;
+            }
+            // Do not let the browser take focus when a Load method has been called
+            return source == CefFocusSource.FocusSourceNavigation;
+        }
+
+        /// <summary>
+        /// Called when the browser component is about to lose focus.
+        /// For instance, if focus was on the last HTML element and the user pressed the TAB key.
+        /// </summary>
+        /// <param name="chromiumWebBrowser">the ChromiumWebBrowser control</param>
+        /// <param name="browser">the browser object</param>
+        /// <param name="next">Will be true if the browser is giving focus to the next component
+        /// and false if the browser is giving focus to the previous component.</param>
+        void IFocusHandler.OnTakeFocus(IWebBrowser chromiumWebBrowser, IBrowser browser, bool next)
+        {
+            //We don't deal with popups as they're rendered by default entirely by CEF
+            if (browser.IsPopup)
+            {
+                return;
+            }
+
+            var winFormsChromiumWebBrowser = (ChromiumWebBrowser)chromiumWebBrowser;
+
+            // NOTE: OnTakeFocus means leaving focus / not taking focus
+            winFormsChromiumWebBrowser.InvokeOnUiThreadIfRequired(() => winFormsChromiumWebBrowser.SelectNextControl(next));
+        }
+    }
+}
+

CefSharp.WinForms.Example/Minimal/MultiFormAppContext.cs

@@ -3,6 +3,7 @@
 // Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
 
 using System.Windows.Forms;
+using CefSharp.WinForms.Example.Handlers;
 
 namespace CefSharp.WinForms.Example.Minimal
 {
@@ -13,9 +14,9 @@ public class MultiFormAppContext : ApplicationContext
 
         public MultiFormAppContext(bool multiThreadedMessageLoop)
         {
-            form1 = new SimpleBrowserForm(multiThreadedMessageLoop);
+            form1 = new SimpleBrowserForm(multiThreadedMessageLoop, new MultiFormFocusHandler());
             form1.WindowState = FormWindowState.Normal;
-            form2 = new SimpleBrowserForm(multiThreadedMessageLoop);
+            form2 = new SimpleBrowserForm(multiThreadedMessageLoop, new MultiFormFocusHandler());
             form2.WindowState = FormWindowState.Normal;
 
             form1.FormClosed += OnFormClosed;

CefSharp.WinForms.Example/Minimal/SimpleBrowserForm.cs

@@ -13,12 +13,14 @@ namespace CefSharp.WinForms.Example.Minimal
     public partial class SimpleBrowserForm : Form
     {
         private ChromiumWebBrowser browser;
+        private IFocusHandler customFocusHandler;
         private bool multiThreadedMessageLoop;
 
-        public SimpleBrowserForm(bool multiThreadedMessageLoop)
+        public SimpleBrowserForm(bool multiThreadedMessageLoop, IFocusHandler customFocusHandler = null)
         {
             InitializeComponent();
 
+            this.customFocusHandler = customFocusHandler;
             this.multiThreadedMessageLoop = multiThreadedMessageLoop;
 
             Text = "CefSharp";
@@ -72,6 +74,12 @@ private void CreateBrowser()
                 browser.FocusHandler = null;
             }
 
+            //Only override if we have a custom handler
+            if (customFocusHandler != null)
+            {
+                browser.FocusHandler = customFocusHandler;
+            }
+
         }
 
         private void OnBrowserConsoleMessage(object sender, ConsoleMessageEventArgs args)

CefSharp.WinForms.Example/Program.cs

@@ -95,6 +95,7 @@ public static int Main(string[] args)
 
                 CefExample.Init(settings, browserProcessHandler: browserProcessHandler);
 
+                //Application.Run(new MultiFormAppContext(multiThreadedMessageLoop));
                 Application.Run(browser);
             }