[gtk/wip/jimmac/scrollbars-fine-tune-fix] Adwaita: avoid fine-tune scrollbars jumping

[Jakub Steiner <jimmac src gnome org>]

commit d7b868a26783b55f0b14895fbce2d318538595f0
Author: Jakub Steiner <jimmac gmail com>
Date:   Mon Feb 15 20:31:44 2021 +0100

    Adwaita: avoid fine-tune scrollbars jumping
    
    Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/3672

 gtk/theme/Adwaita/_common.scss | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)
---
diff --git a/gtk/theme/Adwaita/_common.scss b/gtk/theme/Adwaita/_common.scss
index d084f53ef4..c376571775 100644
--- a/gtk/theme/Adwaita/_common.scss
+++ b/gtk/theme/Adwaita/_common.scss
@@ -2283,6 +2283,7 @@ scrollbar {
   $_slider_min_length: 40px;
   $_slider_width: 8px;
   $_scrollbar_transition: all 300ms $ease-out-quad;
+  $_fine-tune_slider_border: $_slider_width - 3; //3672
 
   background-color: $scrollbar_bg_color;
   transition: $_scrollbar_transition;
@@ -2312,12 +2313,13 @@ scrollbar {
 
   > range.fine-tune {
     > trough > slider {
-      min-width: $_slider_width - 2;
-      min-height: $_slider_width - 2;
+      transition: none;
+      min-width: $_fine-tune_slider_border+1;
+      min-height: $_fine-tune_slider_border+1;
     }
 
-    &.horizontal > trough > slider { border-width: 6px 4px; }
-    &.vertical > trough > slider { border-width: 4px 6px; }
+    &.horizontal > trough > slider { border-width: $_fine-tune_slider_border 4px; }
+    &.vertical > trough > slider { border-width: 4px $_fine-tune_slider_border; }
   }
 
   &.overlay-indicator {