Help me debug (deeply) why clicking a quest it doesn't prioritize it anymore
 

It's like the layer strata is actually detected when the cursor covers over a quest objective (or its circle with a number) but clicking on it does not switch to prioritizing that quest so it makes it unplayable for looking at quest objectives on the map.

I have a lot of addons and a hard time pinpointing which one specifically does it (or a combination of them) so I'd really like some expert debugging info from devs of addons on how they would debug something obscure like that.

I guess you are referring to clicking a quest in the quest tracker?

Does it never work or just not while in combat?

After a relog/reload it does work for a while but then it stops after a while. I have not managed to reproduce it well yet. The little circle on the quest tracker (or the map itself) does give feedback that it's being clicked (it does that modest "click" sound) but it does not select it as prio quest.

After reload, try removing and re-adding a quest to the quest tracker (shift click quest in world map side pane) and then see if your error occurs (in combat or out of combat).

It does it on all quests. This is an issue about clicking any little circle of an objective of a quest and then not being selected as priority for highlighting on map etc.

I did understand! I suspect that there is some taint on the quest tracker entries This can happen, if the quest tracker entries are added from another addon (e.g. a map addon like Mapster oder Leatrix Maps). But you would only notice it during combat lockdown. To rule this out I asked you to perform the steps from my last post.

It's pretty erratic. I saw the problem appearing after I got some dailies (and I hoped I can reproduce it), but after a reload I can't reproduce it by dropping and getting the quest again.

I figured it out, for future reference it can be caused by addons that manipulate the minimap and map. In this case it was FarmHud.