message: make scrollable room list grab available space
  without this, in small sizes, it would make the header shrink.
diff: @@ -131,6 +131,11 @@ limitations under the License.
  transform: rotateZ(-90deg);
  }
  +.mx_RoomSubList .gm-scrollbar-container {
  + /* let rooms list grab all available space */
  + flex: 1;
  +}
  +
  .collapsed {
  .mx_RoomSubList_label {
  height: 17px;
  