In the event that someone in the Debian project wants to fix some bugs, here are a few I've encountered. I was initially using Buster on a 2015 Macbook Pro with the sddm display manager, then I switched to xfce.

0. Mounting an encrypted drive from Dolphin results in numerous notification popups on the lower right-hand side complaining about File Not Found.
1. If the disk gets full, graphical login (sddm) no longer works.
2. If the disk gets full, no GUI warning appears to draw attention to this fact.
3. Switching from sddm to launching X from the command-line and using xfce4, a number of bugs appeared:
- Keyboard brightness up/down keys don't work and cause the backlight to turn off. If I then close the lid and reopen it, the backlight flickers on for maybe 1/4 second and then switches off again.
- Xfce4 menu's lock screen feature does not work.
- Closing the lid of the laptop does not cause sleep.
- Audio volume up/down keys don't work.
4. Wifi connection is often permanently disabled after suspend to RAM, requiring a reboot.
5. Sometimes screen brightness goes higher/lower on its own even though webcam area is covered up.
6. Sometimes after logging in after suspended, the xfce panels all become totally unresponsive.

Reporting it here won't help! Why don't you use bugzilla to report these things directly to the Debian developement team? It should be on your desktop menu somewhere.

additionally
1. in case of disk full almost nothing works (using any distro). It is not a bug. The main reason is: most of the tools attempt to use a tempfile or just write a logfile (or something similar), which is not possible.
2. if you want to report a bug you need to explain how can it be reproduced (just to know what should be fixed and if the solution was correct). see for example here: http://catb.org/~esr/faqs/smart-ques...html#beprecise

Additionally, I would say that since a MacBook Pro was designed specifically to run MacOS and not Debian, that may be the source of some issues (hardware compatibility). Not to say you can't run Debian on a MacBook Pro but it may require tweaks to the installation to get everything to work. Those do not equate to bugs however.

When using Slackware I've encountered zero disk space available but IIRC the graphical login still worked, perhaps because it was based on the very basic xdm.
When writing software it's important to cover not just the expected cases i.e. "the happy path" but also the unexpected and edge cases.
One reason why I created this list is that last time I used either Slackware or Raspbian, I don't recall bugginess on this level. So I suspect this is not going to be solved by making new bug reports but by changing how the Debian project is managed.

That would explain the Wifi behavior but not much else.

You should be so lucky!

including: display a nice dialog about "X cannot be started" ???

Yes, it could say "Oh no! Something has gone wrong!"

What I saw was a black screen. It's a kind of BSOD, by Linux-style.

For comparison on MacOS, which is ostensibly user-centric (except the part about uploading user email data to iCloud even when iCloud is disabled), it does display messages for cases when the drive has less than 3GB free.

Well... I could say you brought it onto yourself or better ask, why you changed distro?

As far as I'm aware, there are no non-resolved bugs in slackware??

And should you find one, I'm pretty certain a request in the forum would quickly sort it...

Reads like you should consider contributing by way of bug reports, or other means if you're able...
https://www.debian.org/doc/manuals/d...buting.en.html

and that works on linux too - the same way (I mean you will get a message on "low disk space"), if
1. notification is set (probably it is the default in most cases)
2. it was noticed
3. disk space was not occupied too fast

That didn't happen with Buster for me. What is the underlying mechanism by which that happens?