Author: Daniel van Vugt <daniel.van.vugt@canonical.com>
Source: https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1915
Editor: Sung Mingi <FiestaLake@protonmail.com>
Commit: 6f195a883c84284c4be802f1a74e425ea1264728
Last Updated: 04/02/22 (gnome-shell 42.0-1)

diff --git a/data/theme/gnome-shell-sass/_common.scss b/data/theme/gnome-shell-sass/_common.scss
index c6e64766e..2b5fccd57 100644
--- a/data/theme/gnome-shell-sass/_common.scss
+++ b/data/theme/gnome-shell-sass/_common.scss
@@ -35,6 +35,11 @@ $base_border_radius: 8px;
 // radii of things that display over other things, e.g. popovers
 $modal_radius: $base_border_radius*2; // 24px
 
+// Chroma key to flag when a background-color is always occluded, not visible.
+// This allows any box-shadow behind it to be rendered more efficiently by
+// omitting the middle rectangle.
+$invisible_occluded_bg_color: rgba(3,2,1,0);
+
 // derived hover colors
 $hover_bg_color: if($variant=='light', darken($bg_color, 3%), lighten($bg_color, 4%));
 $hover_fg_color: if($variant=='light', darken($fg_color, 5%), lighten($fg_color, 4%));
diff --git a/data/theme/gnome-shell-sass/widgets/_window-picker.scss b/data/theme/gnome-shell-sass/widgets/_window-picker.scss
index e494f6d8a..7c3a26cd4 100644
--- a/data/theme/gnome-shell-sass/widgets/_window-picker.scss
+++ b/data/theme/gnome-shell-sass/widgets/_window-picker.scss
@@ -48,5 +48,6 @@ $window_close_button_padding: 3px;
 .workspace-background {
   // keep in sync with BACKGROUND_CORNER_RADIUS_PIXELS in workspace.js
   border-radius: 30px;
+  background-color: $invisible_occluded_bg_color;
   box-shadow: 0 4px 16px 4px transparentize(darken($osd_bg_color, 30%), 0.7);
 }
diff --git a/src/st/st-theme-node-drawing.c b/src/st/st-theme-node-drawing.c
index b32c76be2..72745ed66 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)
     {