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>

diff --git a/src/st/st-theme-node-drawing.c b/src/st/st-theme-node-drawing.c
index b32c76be2fb754e77619e0ca96223e89d087d8ac..72745ed66420a8e8b8aaa2286a4db9c7554800b3 100644 (file)
--- 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)
     {