diff --git a/data/theme/gnome-shell-sass/_common.scss b/data/theme/gnome-shell-sass/_common.scss
index 6372af49e..f2fe78a3c 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);
+
 // fonts
 $base_font_size: 11;
 $text_shadow_color: if($variant == 'light', rgba(255,255,255,0.3), rgba(0,0,0,0.2));
diff --git a/data/theme/gnome-shell-sass/widgets/_window-picker.scss b/data/theme/gnome-shell-sass/widgets/_window-picker.scss
index c71adad54..9067a6d4e 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);
 }