fix viewport changing width when timeline starts filling the viewport

otherwise centered tiles like announcementview jump a bit vertically

src/platform/web/ui/css/timeline.css

@@ -16,7 +16,7 @@ limitations under the License.
 
 
 .RoomView_body > .Timeline {
-    overflow-y: auto;
+    overflow-y: scroll;
     overscroll-behavior-y: contain;
     overflow-anchor: none;
     padding: 0;