[Frontend] Tweaks to scrollbar visibility

Fixes #913
- Scrollbar now always visible in the right side
of the tabular area, in order to avoid column
bottom edges not aligning;

platform/features/timeline/res/sass/_timelines.scss

@@ -58,7 +58,7 @@
                 }
                 &.l-tabular-r {
                     // Start, end, duration, activity modes columns
-                    @include scrollH();
+                    @include scrollH(scroll);
                     left: $timelineTabularTitleW;
 	                .l-width {
 		                @include absPosDefault(0, visible);
@@ -331,4 +331,4 @@
     &:hover {
         background-color: $colorItemTreeHoverBg;
     }
-}
+}