WPF - Add Experimental ChromiumWebBrowserWithMouseCapture

Based on #2871

Use CaptureMouse/ReleaseMouseCapture instead as there's an additional check in ReleaseMouseCapture
https://referencesource.microsoft.com/#PresentationCore/Core/CSharp/System/Windows/UIElement.cs,d6873c5315db5dbb,references

Issue #2258

Author: amaitland

CefSharp.Wpf/CefSharp.Wpf.csproj

@@ -79,6 +79,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="CefSettings.cs" />
+    <Compile Include="Experimental\ChromiumWebBrowserWithMouseCapture.cs" />
     <Compile Include="Experimental\ChromiumWebBrowserWithTouchSupport.cs" />
     <Compile Include="Internals\DragOperationMaskExtensions.cs" />
     <Compile Include="Internals\ImeHandler.cs" />

CefSharp.Wpf/Experimental/ChromiumWebBrowserWithMouseCapture.cs

@@ -0,0 +1,50 @@
+// 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 System;
+using System.Windows.Input;
+
+namespace CefSharp.Wpf.Experimental
+{
+    /// <summary>
+    /// ChromiumWebBrowserWithMouseCapture - Captures the mouse upon left button down to provide
+    /// for dragging the scrollbars/dragging the page down outside of the browser bounds.
+    /// See https://github.com/cefsharp/CefSharp/issues/2258 for more details.
+    /// If using on a Device that has a touch screen you will need to disable WPF StylusAndTouchSupport
+    /// until https://github.com/dotnet/wpf/issues/1323 is resolved.
+    /// </summary>
+    [Obsolete("Only here for testing purposes, will be removed soon as https://github.com/cefsharp/CefSharp/issues/2258 is resolved")]
+    public class ChromiumWebBrowserWithMouseCapture : ChromiumWebBrowser
+    {
+        protected override void OnMouseDown(MouseButtonEventArgs e)
+        {
+            base.OnMouseDown(e);
+
+            //We should only need to capture the left button exiting the browser
+            if (e.ChangedButton == MouseButton.Left && e.LeftButton == MouseButtonState.Pressed)
+            {
+                //Capture/Release Mouse to allow for scrolling outside bounds of browser control (#2870, #2060).
+                //Known issue when capturing and the device has a touch screen, to workaround this issue
+                //disable WPF StylusAndTouchSupport see for details https://github.com/dotnet/wpf/issues/1323#issuecomment-513870984
+                CaptureMouse();
+            }
+        }
+
+        protected override void OnMouseUp(MouseButtonEventArgs e)
+        {
+            base.OnMouseUp(e);
+
+            if (e.ChangedButton == MouseButton.Left && e.LeftButton == MouseButtonState.Released)
+            {
+                //Release the mouse capture that we grabbed on mouse down.
+                //We won't get here if e.g. the right mouse button is pressed and released
+                //while the left is still held, but in that case the left mouse capture seems
+                //to be released implicitly (even without the left mouse SendMouseClickEvent in leave below)
+                //Use ReleaseMouseCapture over Mouse.Capture(null); as it has additional Mouse.Captured == this check
+                ReleaseMouseCapture();
+            }
+        }
+
+    }
+}