Show window on first frame on Linux (#170844)

Wait for the first Flutter frame before showing the window. This avoids
showing a window with empty Flutter content, which defaults to a black
background.

Part of flutter/flutter#55427
Fixes flutter/flutter#118269

Author: robert-ancell

packages/flutter_tools/templates/app/linux.tmpl/runner/my_application.cc.tmpl

@@ -14,6 +14,12 @@ struct _MyApplication {
 
 G_DEFINE_TYPE(MyApplication, my_application, GTK_TYPE_APPLICATION)
 
+// Called when first Flutter frame received.
+static void first_frame_cb(MyApplication* self, FlView *view)
+{
+  gtk_widget_show(gtk_widget_get_toplevel(GTK_WIDGET(view)));
+}
+
 // Implements GApplication::activate.
 static void my_application_activate(GApplication* application) {
   MyApplication* self = MY_APPLICATION(application);
@@ -48,7 +54,6 @@ static void my_application_activate(GApplication* application) {
   }
 
   gtk_window_set_default_size(window, 1280, 720);
-  gtk_widget_show(GTK_WIDGET(window));
 
   g_autoptr(FlDartProject) project = fl_dart_project_new();
   fl_dart_project_set_dart_entrypoint_arguments(project, self->dart_entrypoint_arguments);
@@ -61,6 +66,11 @@ static void my_application_activate(GApplication* application) {
   gtk_widget_show(GTK_WIDGET(view));
   gtk_container_add(GTK_CONTAINER(window), GTK_WIDGET(view));
 
+  // Show the window when Flutter renders.
+  // Requires the view to be realized so we can start rendering.
+  g_signal_connect_swapped(view, "first-frame", G_CALLBACK(first_frame_cb), self);
+  gtk_widget_realize(GTK_WIDGET(view));
+
   fl_register_plugins(FL_PLUGIN_REGISTRY(view));
 
   gtk_widget_grab_focus(GTK_WIDGET(view));