[mutter] clutter/stage-cogl: Colour fb_clip_region in CLUTTER_DEBUG_PAINT_DAMAGE_REGION

[Marge Bot <marge-bot src gnome org>]

commit 96a185d8bc9b904fb253979664ad3e2b7f668eb6
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Wed Nov 18 17:49:59 2020 +0800

    clutter/stage-cogl: Colour fb_clip_region in CLUTTER_DEBUG_PAINT_DAMAGE_REGION
    
    Previously when CLUTTER_DEBUG_PAINT_DAMAGE_REGION was set, that would lead
    to has_buffer_age==FALSE, which would lead to use_clipped_redraw==FALSE
    which would mean swap_region was always empty. And so the blue region of
    CLUTTER_DEBUG_PAINT_DAMAGE_REGION was always empty, *and* fb_clip_region
    was always the full view rectangle which is not useful for debugging.
    
    Now when CLUTTER_DEBUG_PAINT_DAMAGE_REGION is set, we don't let that
    affect use_clipped_redraw, which means fb_clip_region is calculated
    realistically.
    
    But that's not enough. Calculating fb_clip_region properly with
    CLUTTER_DEBUG_PAINT_DAMAGE_REGION would still lead to colouring artefacts
    left on screen from previous frames that don't apply to the current frame.
    So to fix that we also paint_stage for the whole screen every time when
    using CLUTTER_DEBUG_PAINT_DAMAGE_REGION.
    
    So now you will only ever see red and blue shading that's applicable to
    the current frame, and no artefacts from the previous frames.
    
    Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1535
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1571>

 clutter/clutter/cogl/clutter-stage-cogl.c | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)
---
diff --git a/clutter/clutter/cogl/clutter-stage-cogl.c b/clutter/clutter/cogl/clutter-stage-cogl.c
index a5db640b23..92bc6ae519 100644
--- a/clutter/clutter/cogl/clutter-stage-cogl.c
+++ b/clutter/clutter/cogl/clutter-stage-cogl.c
@@ -442,15 +442,6 @@ transform_swap_region_to_onscreen (ClutterStageView *view,
   return transformed_region;
 }
 
-static inline gboolean
-is_buffer_age_enabled (void)
-{
-  /* Buffer age is disabled when running with CLUTTER_PAINT=damage-region,
-   * to ensure the red damage represents the currently damaged area */
-  return !(clutter_paint_debug_flags & CLUTTER_DEBUG_PAINT_DAMAGE_REGION) &&
-         cogl_clutter_winsys_has_feature (COGL_WINSYS_FEATURE_BUFFER_AGE);
-}
-
 static void
 clutter_stage_cogl_redraw_view_primary (ClutterStageCogl *stage_cogl,
                                         ClutterStageView *view)
@@ -484,7 +475,9 @@ clutter_stage_cogl_redraw_view_primary (ClutterStageCogl *stage_cogl,
     COGL_IS_ONSCREEN (onscreen) &&
     cogl_clutter_winsys_has_feature (COGL_WINSYS_FEATURE_SWAP_REGION);
 
-  has_buffer_age = COGL_IS_ONSCREEN (onscreen) && is_buffer_age_enabled ();
+  has_buffer_age =
+    COGL_IS_ONSCREEN (onscreen) &&
+    cogl_clutter_winsys_has_feature (COGL_WINSYS_FEATURE_BUFFER_AGE);
 
   redraw_clip = clutter_stage_view_take_redraw_clip (view);
   if (G_UNLIKELY (clutter_paint_debug_flags & CLUTTER_DEBUG_PAINT_DAMAGE_REGION))
@@ -589,7 +582,15 @@ clutter_stage_cogl_redraw_view_primary (ClutterStageCogl *stage_cogl,
       clutter_damage_history_step (view_priv->damage_history);
     }
 
-  if (use_clipped_redraw)
+  if (clutter_paint_debug_flags & CLUTTER_DEBUG_PAINT_DAMAGE_REGION)
+    {
+      cairo_region_t *debug_redraw_clip;
+
+      debug_redraw_clip = cairo_region_create_rectangle (&view_rect);
+      paint_stage (stage_cogl, view, debug_redraw_clip);
+      cairo_region_destroy (debug_redraw_clip);
+    }
+  else if (use_clipped_redraw)
     {
       cogl_framebuffer_push_region_clip (fb, fb_clip_region);