Mark surface as dirty before flushing it

If we're trying to read back the contents of a GdkWindow that was created from a foreign windowing system surface, we are going to forcibly mark it as dirty before flushing the Cairo state. To avoid regressing any further in the future of the 3.x branch, let's leave a comment. Fixes: #5482

Mark surface as dirty before flushing it
fba09dbf · Emmanuele Bassi · f931c733 · fba09dbf
Commit fba09dbf authored 2 years ago by Emmanuele Bassi
--- a/gdk/gdkpixbuf-drawable.c
+++ b/gdk/gdkpixbuf-drawable.c
@@ -102,8 +102,17 @@ gdk_pixbuf_get_from_window (GdkWindow *src,
  /* We do not know what happened to this surface outside of GDK.
   * Especially for foreign windows, they will have been modified
   * by external applications.
+   *
   * So be on the safe side and:
+   * - mark the surface as dirty, in case the GdkWindow was
+   *   created from a foreign X11 surface
+   * - flush the Cairo state
+   *
+   * For reference, see:
+   * - https://bugzilla.gnome.org/show_bug.cgi?id=754952
+   * - https://gitlab.gnome.org/GNOME/gtk/-/issues/4456
   */
+  cairo_surface_mark_dirty (surface);
  cairo_surface_flush (surface);

  if (cairo_surface_get_content (surface) & CAIRO_CONTENT_ALPHA)