[gtk+] gtk: Keep Firefox working in the DrawingContext world

[Emmanuele Bassi <ebassi src gnome org>]

commit 40ee61a686826ac8e937af720bc152895b10392b
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Wed Jun 22 10:56:27 2016 +0100

    gtk: Keep Firefox working in the DrawingContext world
    
    Firefox does a bunch of interesting things with GTK.
    
    If the top-level GtkWindow does not have a "csd" style class associated,
    Firefox will happily draw the contents of the container used to render
    HTML and XUL directly on the top level's GdkWindow; on the other hand,
    if a "csd" style class is found, the MozContainer will create a new
    child window, and draw on it.
    
    Then, Firefox will proceed to disable double buffering on both the
    top-level window and the MozContainer (unless they are backed by the
    same GdkWindow, in which case only the top-level will be
    single-buffered) *and* it will add a GDK_EXPOSURE_MASK flag to the
    MozContainer events for good measure (even if this is only needed for
    GTK+ 2.x).
    
    After landing the GdkDrawingContext API in GdkWindow, GTK enabled
    automatic double buffering on all top-level windows backed by a native
    surface, ad most users of single buffering rely on child widgets instead
    of top-levels, and we'd still like to have the same double buffering
    behaviour for all top-levels on all backends. Obviously, with Firefox
    disabling double buffering on the top-level window, the change broke
    their drawing mechanism.
    
    Ideally, Firefox could be fixed to not disable double buffering on the
    top-level window when MozContainer has a separate GdkWindow — i.e. the
    CSD case — but since we did introduce a slight change of behaviour in
    fringe users of the GTK+ API, let's keep backwards compatibility with
    the old code for a little while longer, and create an intermediate Cairo
    context unbound from the GdkDrawingContext, like we used to do until
    GTK+ 3.20.

 gtk/gtkwidget.c |   23 +++++++++++++++++++----
 1 files changed, 19 insertions(+), 4 deletions(-)
---
diff --git a/gtk/gtkwidget.c b/gtk/gtkwidget.c
index e6dfac7..5b87a33 100644
--- a/gtk/gtkwidget.c
+++ b/gtk/gtkwidget.c
@@ -17463,15 +17463,30 @@ gtk_widget_render (GtkWidget            *widget,
       /* We only render double buffered on native windows */
       if (!gdk_window_has_native (window))
         return;
-    }
 
-  context = gdk_window_begin_draw_frame (window, region);
-  cr = gdk_drawing_context_get_cairo_context (context);
+      context = gdk_window_begin_draw_frame (window, region);
+      cr = gdk_drawing_context_get_cairo_context (context);
+    }
+  else
+    {
+      /* This is annoying, but it has to stay because Firefox
+       * disables double buffering on a top-level GdkWindow,
+       * which breaks the drawing context.
+       *
+       * Candidate for deletion in the next major API bump.
+       */
+G_GNUC_BEGIN_IGNORE_DEPRECATIONS
+      cr = gdk_cairo_create (window);
+G_GNUC_END_IGNORE_DEPRECATIONS
+    }
 
   do_clip = _gtk_widget_get_translation_to_window (widget, window, &x, &y);
   cairo_translate (cr, -x, -y);
 
   gtk_widget_draw_internal (widget, cr, do_clip);
 
-  gdk_window_end_draw_frame (window, context);
+  if (priv->double_buffered)
+    gdk_window_end_draw_frame (window, context);
+  else
+    cairo_destroy (cr);
 }