Popups are scollable \o/ And the auto complete popup a well.
sublime.COOPERATE_WITH_AUTO_COMPLETE has an interesting side effect where the popup is not closed anymore when text is entered but when the selection is changed through something other than text input or right/left delete, for example by moving left/right or creating a selection. I don't have any issues with that in particular, just mentioning.
It's also displayed on the left of the auto complete popup by default and will move to the right if the popup is would exceed window boundaries otherwise. The traditional way in other editors is to show on the right by default. Just personal preference? I could also see a solution where the popup would go above or below the popup (above still being above the currently edited line), so how about making this an option?
@gregor.hoch: It should be possible to create a context key binding that only triggers when the auto complete popup is visible but I don't think there is a way to get the currently selected item.