make sure the room list doesn't grow taller than viewport

by setting the min-height to 0 at every flex level,
since by default it's auto which means grow as tall as you need
to make the content fit.

res/css/structures/_MatrixChat.scss

@@ -54,6 +54,7 @@ limitations under the License.
     order: 2;
 
     flex: 1;
+    min-height: 0;
 }
 
 .mx_MatrixChat_syncError {