fix: align room timeline scrollbar to viewport edge (#33788)

* fix: align room timeline scrollbar to viewport edge

* update many screenshots

* Lots more screenshots

* screenshots

* Manual screenshot update

---------

Co-authored-by: David Baker <dbkr@users.noreply.github.com>

apps/web/res/css/structures/_RoomView.pcss

@@ -97,10 +97,16 @@ Please see LICENSE files in the repository root for full details.
         flex: 1;
         display: flex;
         flex-direction: column;
-        margin-right: calc(var(--container-gap-width) / 2);
     }
 }
 
+/* Restore the gap between the timeline and the right panel only when the panel is open.
+   Without this, the scrollbar on mx_RoomView_messagePanel would sit 4 px away from the
+   viewport edge in Firefox (which does not support overflow:overlay). */
+.mx_MainSplit:has(.mx_RightPanel_ResizeWrapper) .mx_RoomView_timeline {
+    margin-right: calc(var(--container-gap-width) / 2);
+}
+
 .mx_RoomView_statusArea {
     width: 100%;
     flex: 0 0 auto;