Reposition window after window show, just in case

The GTK documentation on `gtk_window_move` [1] state this:

> Asks the window manager to move window to the given position. Window
> managers are free to ignore this; most window managers ignore requests
> for initial window positions (instead using a user-defined placement
> algorithm) and honor requests after the window has already been shown.

That may be what causes the overlay to the positioned wrong in #18 and
#20 (comment). In any
case, it should do no harm to make sure.

[1]: https://docs.gtk.org/gtk3/method.Window.move.html

main.c

@@ -34,6 +34,7 @@ static void screen_changed(GtkWidget *widget, GdkScreen *old_screen,
 static void composited_changed(GdkScreen *screen, gpointer user_data);
 static void on_close_web_view(WebKitWebView *web_view, gpointer user_data);
 
+static void size_to_screen(GtkWindow *window);
 static int get_monitor_rects(GdkDisplay *display, GdkRectangle **rectangles) {
     int n = gdk_display_get_n_monitors(display);
     GdkRectangle *new_rectangles = (GdkRectangle*)malloc(n * sizeof(GdkRectangle));
@@ -784,6 +785,15 @@ int main(int argc, char **argv) {
     // the WM should ignore it.
     gdk_window_show(GDK_WINDOW(gdk_window));
 
+    // Move window to match monitor layout.  This should already have been done
+    // by `screen_changed` above, but we repeat it here after
+    // `gdk_window_show`, in case the running window manager applies its own
+    // overriding rules for initial window positioning when a window becomes
+    // visible.  This could cause a few frames of the wrong window position
+    // being shown on affected window managers, but should do nothing on window
+    // managers that behave properly.
+    size_to_screen(GTK_WINDOW(window));
+
     //
     // Set up the JavaScript API
     //