use AutoHideScrollbar in RoomSubList

res/css/views/rooms/_RoomList.scss

@@ -17,7 +17,7 @@ limitations under the License.
 
 .mx_RoomList {
     /* take up remaining space below TopLeftMenu */
-    flex: 1;
+    flex: 1 1 auto;
     /* use flexbox to layout sublists */
     display: flex;
     flex-direction: column;

res/css/views/rooms/_RoomTile.scss

@@ -20,7 +20,7 @@ limitations under the License.
     align-items: center;
     cursor: pointer;
     height: 40px;
-    margin: 0 12px;
+    margin: 0;
     padding: 2px 12px;
     position: relative;
     background-color: $secondary-accent-color;