Fix webview rendering on _NET_WM_FRAME_DRAWN WMs

Author: maxkatz6

src/Avalonia.Controls.WebView.Core/Gtk/GtkInterop.cs

@@ -339,6 +339,10 @@ internal static partial IntPtr webkit_website_data_manager_new(
     [DllImport(LibGdk)]
     internal static extern IntPtr gdk_x11_window_get_xid(IntPtr window);
 
+    [DllImport(LibGdk)]
+    internal static extern void gdk_x11_window_set_frame_sync_enabled(IntPtr window,
+        [MarshalAs(UnmanagedType.Bool)] bool frameSyncEnabled);
+
     [DllImport(LibGdk)]
     internal static extern IntPtr gdk_display_get_default();
     [DllImport(LibGdk)]

src/Avalonia.Controls.WebView.Core/Gtk/GtkX11WebViewAdapter.cs

@@ -23,7 +23,14 @@ private GtkX11WebViewAdapter(GtkWebViewEnvironmentRequestedEventArgs environment
         gtk_container_add(_windowHandle, WebViewHandle);
         gtk_widget_show_all(WebViewHandle);
         gtk_widget_realize(_windowHandle);
-        _x11Window = gdk_x11_window_get_xid(gtk_widget_get_window(_windowHandle));
+
+        var gdkWindow = gtk_widget_get_window(_windowHandle);
+        _x11Window = gdk_x11_window_get_xid(gdkWindow);
+
+        // Reparenting this window into the Avalonia window takes it away from the window manager,
+        // so the _NET_WM_FRAME_DRAWN replies GDK throttles drawing on never arrive.
+        // Without this GTK paints one frame and then waits forever.
+        gdk_x11_window_set_frame_sync_enabled(gdkWindow, false);
     }
 
     public static Task<WebViewAdapter.NativeWebViewAdapterBuilder> CreateBuilder(