[gnome-shell] st/scroll-view-fade: Fix vertical top fading

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

commit 2501bc5c8f97581a3ea1bb141feb5c721a316de7
Author: Sebastian Keller <skeller gnome org>
Date:   Sat Feb 13 21:13:57 2021 +0100

    st/scroll-view-fade: Fix vertical top fading
    
    The fade for the vertical top edge was calculating the fade ratio for a
    larger height (up to where the bottom fade starts) than it was
    displaying.
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1674>

 src/st/st-scroll-view-fade.glsl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/st/st-scroll-view-fade.glsl b/src/st/st-scroll-view-fade.glsl
index 2eacfd9af5..e3f1a259f3 100644
--- a/src/st/st-scroll-view-fade.glsl
+++ b/src/st/st-scroll-view-fade.glsl
@@ -55,7 +55,7 @@ void main ()
 
         float vfade_scale = height / vfade_offset;
         if (fade_top) {
-            ratio *= (fade_area_topleft[1] - y) / (fade_area_topleft[1] - fade_bottom_start);
+            ratio *= (fade_area_topleft[1] - y) / (fade_area_topleft[1] - fade_top_start);
         }
 
         if (fade_bottom) {