diff --git a/gdk/gdkrectangleprivate.h b/gdk/gdkrectangleprivate.h
index b7857d5037..3742764a41 100644
--- a/gdk/gdkrectangleprivate.h
+++ b/gdk/gdkrectangleprivate.h
@@ -52,6 +52,16 @@ gdk_rectangle_transform_affine (const GdkRectangle *src,
   dest->height = ceilf (MAX (y1, y2)) - dest->y;
 }
 
+static inline gboolean
+gdk_rectangle_contains (const GdkRectangle *rect,
+                        const GdkRectangle *contained)
+{
+  return contained->x >= rect->x
+      && contained->y >= rect->y
+      && contained->x + contained->width <= rect->x + rect->width
+      && contained->y + contained->height <= rect->y + rect->height;
+}
+
 
 G_END_DECLS
 
diff --git a/gdk/gdksurface.c b/gdk/gdksurface.c
index ef444b123b..ddde1fd427 100644
--- a/gdk/gdksurface.c
+++ b/gdk/gdksurface.c
@@ -40,7 +40,7 @@
 #include <glib/gi18n-lib.h>
 #include "gdkmarshalers.h"
 #include "gdkpopupprivate.h"
-#include "gdkrectangle.h"
+#include "gdkrectangleprivate.h"
 #include "gdktoplevelprivate.h"
 #include "gdkvulkancontext.h"
 #include "gdksubsurfaceprivate.h"
@@ -2738,6 +2738,33 @@ gdk_surface_set_opaque_rect (GdkSurface            *self,
   gdk_surface_update_opaque_region (self);
 }
 
+/*
+ * gdk_surface_is_opaque:
+ * @self: a surface
+ *
+ * Checks if the whole surface is known to be opaque.
+ * This allows using an RGBx buffer instead of RGBA.
+ *
+ * This function works for the currently rendered frame inside
+ * begin_frame() implementations.
+ *
+ * Returns: %TRUE if the whole surface is provably opaque
+ **/
+gboolean
+gdk_surface_is_opaque (GdkSurface *self)
+{
+  GdkSurfacePrivate *priv = gdk_surface_get_instance_private (self);
+  cairo_rectangle_int_t whole = { 0, 0, self->width, self->height };
+
+  if (gdk_rectangle_contains (&priv->opaque_rect, &whole))
+    return TRUE;
+
+  if (cairo_region_contains_rectangle (priv->opaque_region, &whole) == CAIRO_REGION_OVERLAP_IN)
+    return TRUE;
+
+  return FALSE;
+}
+
 void
 gdk_surface_set_state (GdkSurface      *surface,
                        GdkToplevelState new_state)
diff --git a/gdk/gdksurfaceprivate.h b/gdk/gdksurfaceprivate.h
index 6a5353a3d3..04c8a39524 100644
--- a/gdk/gdksurfaceprivate.h
+++ b/gdk/gdksurfaceprivate.h
@@ -261,6 +261,7 @@ void       _gdk_surface_clear_update_area (GdkSurface            *surface);
 void       _gdk_surface_update_size       (GdkSurface            *surface);
 void       gdk_surface_set_opaque_rect    (GdkSurface            *self,
                                            const graphene_rect_t *rect);
+gboolean   gdk_surface_is_opaque          (GdkSurface            *self);
 
 GdkGLContext * gdk_surface_get_paint_gl_context (GdkSurface *surface,
                                                  GError   **error);
diff --git a/gdk/macos/gdkmacosglcontext.c b/gdk/macos/gdkmacosglcontext.c
index 86c2cc9015..039c4395bc 100644
--- a/gdk/macos/gdkmacosglcontext.c
+++ b/gdk/macos/gdkmacosglcontext.c
@@ -242,7 +242,7 @@ gdk_macos_gl_context_allocate (GdkMacosGLContext *self)
     return;
 
   /* Alter to an opaque surface if necessary */
-  opaque = _gdk_macos_surface_is_opaque (GDK_MACOS_SURFACE (surface));
+  opaque = gdk_surface_is_opaque (surface);
   if (opaque != self->last_opaque)
     {
       self->last_opaque = !!opaque;
diff --git a/gdk/macos/gdkmacossurface-private.h b/gdk/macos/gdkmacossurface-private.h
index ff0f297a8a..39d1ff28d8 100644
--- a/gdk/macos/gdkmacossurface-private.h
+++ b/gdk/macos/gdkmacossurface-private.h
@@ -86,7 +86,6 @@ CGDirectDisplayID  _gdk_macos_surface_get_screen_id           (GdkMacosSurface
 const char        *_gdk_macos_surface_get_title               (GdkMacosSurface      *self);
 void               _gdk_macos_surface_set_title               (GdkMacosSurface      *self,
                                                                const char           *title);
-gboolean           _gdk_macos_surface_is_opaque               (GdkMacosSurface      *self);
 NSView            *_gdk_macos_surface_get_view                (GdkMacosSurface      *self);
 gboolean           _gdk_macos_surface_get_modal_hint          (GdkMacosSurface      *self);
 void               _gdk_macos_surface_set_modal_hint          (GdkMacosSurface      *self,
diff --git a/gdk/macos/gdkmacossurface.c b/gdk/macos/gdkmacossurface.c
index eb167ec391..f4d50be92c 100644
--- a/gdk/macos/gdkmacossurface.c
+++ b/gdk/macos/gdkmacossurface.c
@@ -553,29 +553,6 @@ gdk_macos_surface_init (GdkMacosSurface *self)
   self->monitors = g_ptr_array_new_with_free_func (g_object_unref);
 }
 
-gboolean
-_gdk_macos_surface_is_opaque (GdkMacosSurface *self)
-{
-  GdkSurface *surface = (GdkSurface *)self;
-
-  g_return_val_if_fail (GDK_IS_MACOS_SURFACE (self), FALSE);
-
-  if (surface->opaque_region != NULL &&
-      cairo_region_num_rectangles (surface->opaque_region) == 1)
-    {
-      cairo_rectangle_int_t extents;
-
-      cairo_region_get_extents (surface->opaque_region, &extents);
-
-      return (extents.x == 0 &&
-              extents.y == 0 &&
-              extents.width == GDK_SURFACE (self)->width &&
-              extents.height == GDK_SURFACE (self)->height);
-    }
-
-  return FALSE;
-}
-
 const char *
 _gdk_macos_surface_get_title (GdkMacosSurface *self)
 {