[gnome-shell] st/theme-node-drawing: Conditionally paint the center shadow rectangle

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

commit 9b3ee774f622c6ac1d4857d6ae6dbe78d475dfe0
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Tue May 25 17:33:01 2021 +0800

    st/theme-node-drawing: Conditionally paint the center shadow rectangle
    
    Only when part of it is actually visible.
    
    Because the central rectangle is generally the largest part, this
    eliminates most of the shadow's render time. For example, animating
    `.workspace-background` by tapping Super, the shell's overall render
    time is reduced about 15%.
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1915>

 src/st/st-theme-node-drawing.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)
---
diff --git a/src/st/st-theme-node-drawing.c b/src/st/st-theme-node-drawing.c
index b32c76be2f..72745ed664 100644
--- a/src/st/st-theme-node-drawing.c
+++ b/src/st/st-theme-node-drawing.c
@@ -2021,6 +2021,8 @@ st_theme_node_paint_sliced_shadow (StThemeNodePaintState *state,
   gfloat shadow_blur_radius, x_spread_factor, y_spread_factor;
   float rectangles[8 * 9];
   gint idx;
+  ClutterColor background_color;
+  static const ClutterColor invisible_occluded = {3, 2, 1, 0};
 
   if (paint_opacity == 0)
     return;
@@ -2178,16 +2180,23 @@ st_theme_node_paint_sliced_shadow (StThemeNodePaintState *state,
       rectangles[idx++] = s_bottom;
     }
 
-  /* Center middle */
-  rectangles[idx++] = left;
-  rectangles[idx++] = top;
-  rectangles[idx++] = right;
-  rectangles[idx++] = bottom;
+  /* Center middle is not definitely occluded? */
+  st_theme_node_get_background_color (node, &background_color);
+  if (!clutter_color_equal (&background_color, &invisible_occluded) ||
+      paint_opacity < 255 ||
+      xoffset > shadow_blur_radius || left < 0 ||
+      yoffset > shadow_blur_radius || top < 0)
+    {
+      rectangles[idx++] = left;
+      rectangles[idx++] = top;
+      rectangles[idx++] = right;
+      rectangles[idx++] = bottom;
 
-  rectangles[idx++] = s_left;
-  rectangles[idx++] = s_top;
-  rectangles[idx++] = s_right;
-  rectangles[idx++] = s_bottom;
+      rectangles[idx++] = s_left;
+      rectangles[idx++] = s_top;
+      rectangles[idx++] = s_right;
+      rectangles[idx++] = s_bottom;
+    }
 
   if (xend > right)
     {