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/_LeftPanel.scss

@@ -51,6 +51,7 @@ limitations under the License.
     overflow-x: hidden;
     display: flex;
     flex-direction: column;
+    min-height: 0;
 }
 
 .mx_LeftPanel .mx_AppTile_mini {

res/css/structures/_MatrixChat.scss

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

res/css/views/rooms/_RoomList.scss

@@ -21,6 +21,7 @@ limitations under the License.
     /* use flexbox to layout sublists */
     display: flex;
     flex-direction: column;
+    min-height: 0;
 }
 
 /* hide resize handles next to collapsed / empty sublists */