i've filed a bug report but it seems that it's limited to the desktop manager: with XFCE I had noticed the disappearance of the scroll sliders everywhere in AOO (main window, sidebar, help files, dialogs, ...):
Xubuntu use XFCE. Have you changed the window manager?
I made some tests, (I was mainly waiting for your answer in fact since I know you use Xubuntu) and found out that it was a problem with the themes. Some of the themes have this problem and some don't...
Thanks! I should have tried that earlier.
LibreOffice 7.6.2.1 on Xubuntu 23.10 and 7.6.4.1 portable on Windows 10
In fact, after checking some old mail from the dev ML, I saw that this had been reported already. I had not used the correct keywords to search the bugzilla (scrollbar is the good one...).
-> Issue 121627 - [GTK] Scrollbars without steppers are buggy
LibreOffice 7.6.2.1 on Xubuntu 23.10 and 7.6.4.1 portable on Windows 10
There is so little difference between many of the Xubuntu themes (leaving aside the dark and high contrast ones) that after ten minutes I doubt you'd notice you were running Bluebird!
But I hate the blue background for the menus, I prefer the black background for menus. I've found a nice theme: Ambiance light fixed. Have tweaked the base background to have a very light gray instead of white and I'm fine with that.
LibreOffice 7.6.2.1 on Xubuntu 23.10 and 7.6.4.1 portable on Windows 10