Auto-completion (of words previously typed, member function suggestions, etc.) is both useful and annoying. It is useful because it saves typing. It is annoying because it visually hides other parts of the document, distracts, and blocks keys (worst when you’re not looking at the screen and the auto-completion means or an arrow key doesn’t do what you expect).
Use dedicated keys. Making all keys like arrows, space, enter and tab work as usual even when suggestions are shown would remove some of the annoyance. Keyboards have a bunch of function keys; use those! E.g. F1/F2 (or whatever) to activate the previous/next suggestion and escape or any keyboard input closing the completions box (finalising selection). Since using F1/F2 or whatever isn’t obvious, just write something like “F1: previous, F2: next” at the bottom of the pop-up box.
Make the suggestions box transparent. Not being able to see what’s behind the suggestions box can be annoying, so make that possible (e.g. 20% transparency of pop-up box’s background).
Yes, just a pet peeve, but making those things less annoying really wouldn’t be so difficult!