Create WPF_WebView2CompositionControl

Author: champnic

specs/WPF_WebView2CompositionControl

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+
+WPF Airspace - WebView2CompositionControl
+===
+
+# Background
+For a long time, the WebView2 WPF control has had airspace issues, where the WebView2 control will always
+be the top-most control in the WPF app. Any WPF element in the same location as the WebView2 control will
+end up below the WebView2 control and be "invisible" to the end-user. This issue stems from the fact that
+our WPF control uses the WPF HwndHost to host the Win32 WebView2 control, and HwndHost has a long-standing
+issue with airspace. This issue has existed for over a decade, and you can read more about in-depth efforts to fix
+it in general in WPF here:
+https://dwayneneed.github.io/wpf/2013/02/26/mitigating-airspace-issues-in-wpf-applications.html
+
+Solving this issue for WebView2 necessitates moving away from the HwndHost and the windowed hosting model
+of WebView2, and instead use visuals to host the WebView2. To avoid compatibility issues and introducing
+new regressions in the existing control, we decided to create a new WebView2 WPF control, called the WebView2CompositionControl.
+
+# Examples
+In this example we create a WebView2CompositionControl in exactly the same way we would a regular
+WebView2 control in WPF. Additionally, we place a pink Label that appears ABOVE the WebView2CompositionControl.
+If you were to instead do this with the regular WebView2 control, the Label would be hidden behind it.
+```xml
+    <DockPanel>
+
+        <DockPanel
+            DockPanel.Dock="Top">
+            <Button
+                x:Name="ButtonGo"
+                DockPanel.Dock="Right"
+                Click="ButtonGo_Click"
+                Content="Go" />
+            <TextBox Name="addressBar"/>
+        </DockPanel>
+        <Grid>
+            <wv2:WebView2CompositionControl
+                Name="webView"
+                Source="https://www.microsoft.com"
+                AllowExternalDrop="False"/>
+            <Label
+                Content="This is a label over the WebView2!"
+                Background="LightPink"
+                Height="100"
+                Width="300"/>
+        </Grid>
+    </DockPanel>
+```
+
+
+# API Details
+If you are building a Windows Presentation Foundation (WPF) app and using the WebView2 control, you
+may find that your app runs into "airspace" issues, where the WebView2 control will always show up
+on top and hide any WPF elements in the same location, even if you try to specify the WPF elements
+to be above the WebView2 control (using visual tree order or the z-index property, for example). To
+solve this issue you should use the WebView2CompositionControl. 
+
+The WebView2CompositionControl is a drop-in replacement
+for the standard WPF WebView2 control - both the WebView2 control and the WebView2CompositionControl
+implement the [Microsoft.Web.WebView2.Wpf.IWebView2](https://learn.microsoft.com/en-us/dotnet/api/microsoft.web.webview2.wpf.iwebview2?view=webview2-dotnet-1.0.2739.15)
+interface, as well as derive from FrameworkElement (`FrameworkElement -> HwndHost -> WebView2`, and
+`FrameworkElement -> Control -> WebView2CompositionControl`). So anywhere you use
+WebView2, you can instead change to a WebView2CompositionControl without changing your
+code.
+
+We chose the name "WebView2CompositionControl" because it matches our naming convention
+for visual hosting WebView2, which uses the CoreWebView2CompositionController instead
+of the CoreWebView2Controller.
+
+# Appendix
+At a high level, the WebView2CompositionControl instantiates a CoreWebView2 in much the same way as
+the regular WebView2 control, but connects to it and controls it using the CoreWebView2CompositionController
+instead of the CoreWebView2Controller. The composition controller is given a WinComp visual to draw into,
+and this visual is used to create a GraphicsCaptureSession. When frames from this capture session are
+captured they are they rendered into a WPF Image element that's part of the control.