bring back global style rules for thin scrollbars (also FF now!)

res/css/structures/_AutoHideScrollbar.scss

@@ -14,6 +14,41 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+// make any scrollbar grey and thin
+html {
+    scrollbar-color: $scrollbar-thumb-color $scrollbar-track-color;
+}
+// scrollbar-width is not inherited (but -color is, why?!),
+// so declare it on every element
+* {
+    scrollbar-width: thin;
+}
+
+::-webkit-scrollbar {
+    width: 6px;
+    height: 6px;
+    background-color: $scrollbar-track-color;
+}
+
+::-webkit-scrollbar-thumb {
+    border-radius: 3px;
+    background-color: $scrollbar-thumb-color;
+}
+
+// make auto-hide scrollbars not transparent again on hover
+.mx_AutoHideScrollbar:hover {
+    scrollbar-color: $scrollbar-thumb-color $scrollbar-track-color;
+
+    &::-webkit-scrollbar {
+        background-color: $scrollbar-track-color;
+    }
+
+    &::-webkit-scrollbar-thumb {
+        background-color: $scrollbar-thumb-color;
+    }
+}
+
+// make scrollbars transparent for autohide scrollbars
 .mx_AutoHideScrollbar {
     overflow-x: hidden;
     overflow-y: auto;
@@ -21,27 +56,12 @@ limitations under the License.
     -ms-overflow-style: -ms-autohiding-scrollbar;
 
     &::-webkit-scrollbar {
-        width: 6px;
-        height: 6px;
         background-color: transparent;
     }
 
     &::-webkit-scrollbar-thumb {
-        border-radius: 3px;
         background-color: transparent;
     }
 
     scrollbar-color: transparent transparent;
-    scrollbar-width: thin;
-}
-
-.mx_AutoHideScrollbar:hover {
-    &::-webkit-scrollbar {
-        background-color: $scrollbar-track-color;
-    }
-
-    &::-webkit-scrollbar-thumb {
-        background-color: $scrollbar-thumb-color;
-    }
-    scrollbar-color: $scrollbar-thumb-color $scrollbar-track-color;
 }