Attempt to support a hard cutoff with the show more button

Known issues:
* Causes scroll jumps when the button gets added to DOM
* Resize handle is invisible when there's a show more button

TODO:
* Clean up comments
* Clean up useless code (all the padding stuff isn't needed)

src/stores/room-list/ListLayout.ts

@@ -20,7 +20,8 @@ const TILE_HEIGHT_PX = 44;
 
 // this comes from the CSS where the show more button is
 // mathematically this percent of a tile when floating.
-const RESIZER_BOX_FACTOR = 0.78;
+//const RESIZER_BOX_FACTOR = 0.78;
+const RESIZER_BOX_FACTOR = 0;
 
 interface ISerializedListLayout {
     numTiles: number;