SUMMARY

The filesystem tree view is broken. Folders can be expanded if they have folders within them, but not if they only have files within them. This makes it impossible to open files in bottom-level folders using the filesystem tree.

There is a pretty good description here:
https://forum.kde.org/viewtopic.php?f=218&t=165464
(which was not written by me).

STEPS TO REPRODUCE
1. Open KDevelop
2. Try to expand a bottom-level folder (i.e. one without further folders within it)

OBSERVED RESULT
No way of expanding the folder (no little expander caret symbol)

EXPECTED RESULT
Expander caret to allow folder to be expanded, so that files within can be opened.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 20.04
(available in About System)
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION
None