I had a problem with the tab button. When I pressed it it would cycle through the UI elements instead of adding spaces; though, tab-completion worked.
While searching I found a post on this site with the same problem, but no answer: Issue: tab to insert spaces does not work - #2 by petestyl
I found out there is a keyboard shortcut that toggles tab functionality. It's labeled as "Toggle Tab Key Moves Focus" and is activated with "Alt + Shift + [".