matrix-react-sdk

ffcba949 - Keep context menus that extend downwards vertically on screen

Keep context menus that extend downwards vertically on screen This fixes https://github.com/vector-im/riot-web/issues/3429 in the case of RoomList context menus *and* EventTile context menus. This was mostly stolen from #1903 and it would be good to factor things out at a later date such that #1903 is replaced with similar logic within ContextualMenu.