As seen here, it looks like Questie is tracking more quests than it is supposed to. I am only tracking 3 quests, but if I right click anywhere on the DPS meter it acts as if more quests were tracked. This is about how it looks with all my quests tracked (quest tracker and DPS meter are not in the same position as the previous picture, but that's not the point)

I also encountered this bug (it messed up interaction with the top of my inventory) so I pulled the source and had a look. Seems like the code only hides the labels, not the GUI elements themselves when untracking quests. Don't know if this is intentional or not.

Applied the following patch to fix the issue:
1174_patch.txt

Good stuff, thanks @throwawaydev2 and @AeroScripts