Currently, the custom games list auto refreshes at certain intervals (or so it would appear).
If you select a game (so it is highlighted), and an auto refresh occurs before you are able to click the 'join game' button - the selection is lost. This selection seems to be lost regardless of whether or not the auto refresh caused the selected item to move up or down the page.
It seems obvious this is not how it is intended to work, as otherwise there would be no 'join game' button (which is reliant on a selection), and only the 'double click' option. The double click option has an element of risk, as sometimes the game you are double clicking and shift positions mid-click as a result of auto refresh, and you end up in the wrong game.
Cheers!