- Set NativeWebDialog.UserAgent in AdapterCreated instead of passing a full
User-Agent header to ApplicationNameForUserAgent (which caused WebKit2GTK
webkit_settings_set_user_agent assertion failures on Linux).
- Set gtkArgs.ForceX11GdkBackend = true when requesting the GTK WebView
environment.
- Recognize GTK initialization failures (InvalidOperationException 'Unable to
initialize GTK', libgtk-3/libgdk-3 DllNotFoundException, and exceptions
originating from GtkNativeWebViewDialog/AvaloniaGtk) in
WebView2LoginErrorMessage so async void NativeWebDialog.Show() failures
fault BrowserLoginAsync instead of leaving tcs.Task waiting forever,
allowing StartAsyncInternal to fall back to LoginExternalDialog.