Seems to be fixed in VLC 22.214.171.124-1 (Ubuntu 20.04); at least I haven’t been able to reproduce it anymore.
Viestit paikassa Launchpad
Same ”double free or corruption (!prev)” reproducible here too (VLC 126.96.36.199-1, Ubuntu 20.04), even more easily: the position doesn’t matter, and the first attempt to screenshot is already enough to trigger the issue.
I came across this when trying to screenshot a Superman cartoon I downloaded from Internet Archive .
*  https://archive.org/download/dom-6746superman-episode7-theundergroundworld512kb/dom-6746superman-episode7-theundergroundworld512kb.mp4
There’s no gnome-shell-extension-dashtodock package for focal , but I did try with Dash to Dock from extensions.gnome.org, and could not reproduce the issue with it. I tried with the default settings, as well as after recreating the look & feel of Ubuntu Dock as much as possible.
Dash to Dock has customizable action for the scroll (to either switch workspaces or windows, or to do nothing), and all of those also worked just as expected.
I also noticed that using the two finger scroll function of the touchpad on my laptop does *not* trigger this (whereas using the mouse on it does).
*  https://packages.ubuntu.com/search?keywords=gnome-shell-extension-dashtodock
Do you want another log for that? Because (as I already mentioned above), this also reproduces when using a newly-created user, so that means no unsupported extensions.
I can reproduce the crash at will with the steps I listed above, so I don’t have to wait for it to happen. It also doesn’t forcibly log me out, so I can enter those commands by just having a terminal window present when I trigger the issue. But I logged out here (after triggering) anyway since you instructed me to log in, in case it matters.
Also, there’s lots of noise in these from Boinc, thanks to #1876313.
1. There’s nothing in /var/crash
2. There are no reports on the page for this computer.
3. Done, still nothing in /var/crash.
I don’t know if this means it’s not a true crash, although visually all the indications are there.
I also verified that this reproduces when using a newly-created user (with the only change from defaults for it being switching to Wayland), and also on another computer (also running 20.04). I am limited to using the one mouse that I have, so that all testing so far has been with the same mouse.
# Steps to reproduce
0. have a mouse with horizontal scrolling functionality (mine is a Microsoft Comfort Mouse 4500 with a tilting action for horizontal scroll)
1. move mouse pointer over the dock
2. use the mouse scroller to scroll horizontally
# What I expect to happen
# What happens
Mouse pointer and everything on screen freezes. After a few seconds it unfreezes, but all the icons from the dock are missing. I have to log out and back in to restore the dock.
# Other info
Slightly similar to LP #1875106, but I’m not using imwheel.
I am using Wayland.
Syslog seems to always point to this one function: ”The offending callback was get_preferred_height(), a vfunc”. I’ll attach the relevant part from one crash.
No, I haven’t (and wouldn’t know how).
Yes, this is still present in 20.04 with printer-driver-cups-pdf 3.0.1-6, limited to original reporter’s case of having the symlink point to another filesystem; if I point ~/PDF to, say ~/tmp/PDF (without crossing filesystem borders), the PDF is created without issue, whereas having ~/PDF linked to /media/$USER/$USB_THUMBDRIVE/PDF/ does still cause it to fail:
Sat Dec 26 16:48:07 2020 [DEBUG] output filename created: /home/jani/PDF/Bug__1092099__342_200_234inconsistent_behavior_for…ry_342_200_-job_8.pdf
Sat Dec 26 16:48:07 2020 [DEBUG] ghostscript commandline built: /usr/bin/gs -q -dCompatibilityLevel=1.2 -dNOPAUSE -dBATCH -dSAFER -sDEVICE=pdfwrite -sOutputFile=”/home/jani/PDF/Bug__1092099__342_200_234inconsistent_behavior_for…ry_342_200_-job_8.pdf” -dAutoRotatePages=/PageByPage -dAutoFilterColorImages=false -dColorImageFilter=/FlateEncode -dPDFSETTINGS=/prepress -c .setpdfwrite -f /var/spool/cups-pdf/SPOOL/cups2pdf-233705
Sat Dec 26 16:48:07 2020 [DEBUG] output file unlinked: /home/jani/PDF/Bug__1092099__342_200_234inconsistent_behavior_for…ry_342_200_-job_8.pdf
Sat Dec 26 16:48:07 2020 [DEBUG] TMPDIR set for GhostScript: /var/tmp
Sat Dec 26 16:48:07 2020 [DEBUG] waiting for child to exit
Sat Dec 26 16:48:07 2020 [DEBUG] entering child process
Sat Dec 26 16:48:07 2020 [DEBUG] GID set for current user
Sat Dec 26 16:48:07 2020 [DEBUG] supplementary groups set for current user
Sat Dec 26 16:48:07 2020 [DEBUG] UID set for current user: jani
Sat Dec 26 16:48:07 2020 [DEBUG] ghostscript has finished: 256
Sat Dec 26 16:48:07 2020 [ERROR] failed to set file mode for PDF file: /home/jani/PDF/Bug__1092099__342_200_234inconsistent_behavior_for…ry_342_200_-job_8.pdf (non fatal)
Sat Dec 26 16:48:07 2020 [DEBUG] ERRNO: 2 (No such file or directory)
Sat Dec 26 16:48:07 2020 [DEBUG] no postprocessing
Sat Dec 26 16:48:07 2020 [DEBUG] spoolfile unlinked: /var/spool/cups-pdf/SPOOL/cups2pdf-233705
Sat Dec 26 16:48:07 2020 [DEBUG] all memory has been freed
Sat Dec 26 16:48:07 2020 [STATUS] PDF creation successfully finished for jani
jani@latimeria:~$ ls -l PDF
lrwxrwxrwx 1 jani jani 27 joulu 26 16:47 PDF -> /media/jani/KINGSTON_1G/PDF
jani@latimeria:~$ ls -l PDF/
Sorry, tried to link this with LP #1723881 but failed, because LP #1334