Dear all, maybe someone is around who can help me with my question:

Why does Baloo need access to X11?

The background: Some of you may have heard already of Firejail, which is an application sandboxing tool. For many applications Firejail comes with pre-written and tested profiles, which makes it pretty easy to use for beginners. Even writing these profiles is usually not a difficult thing, but there are exceptions..... and this brings us to Baloo.
Baloo, as a daemon, shouldn't care at all about X11. At least that was what I thought. In order to isolate it from GUI-applications, I blocked the access to /tmp/.X11-unix via Firejail, and observed to my surprise that as a result Baloo's indexing was very reproducibly stuck at a certain low file number. Note that Baloo was instructed to only index the home directory, and restarting Baloo/resetting the database didn't help at all..... in the end what helped was unblocking /tmp/.X11-unix!

Then I learned I was able to fool Baloo by using the untrusted mode of the X11 security extension (via Firejail). Which means I give it some access to X11, but it is unable to see what the other GUI applications are doing. But now my concern is that Baloo will fool me in the future, in a moment I expect it the least So it would be great to have a coarse understanding what is going on.