See video: https://gfycat.com/LimpJointAcouchi Plasma version: 5.15.0 and older KDE Frameworks Version: 5.55 and older Qt Version: 5.12.1 and older ADDITIONAL INFORMATION This bug appears very rarely out-of-the-blue and is gone after reboot or relogin (plasmashell process re-launch). On both Xorg and Wayland. This is also not a new bug in Plasma 5.15, it has been here for a while. I don't know what triggers this bug, and also I don't know how we call the expanded applet area, which I named 'the arrow'.
This was fixed in: https://bugs.kde.org/show_bug.cgi?id=393630 If you don't want to wait until your distro ships it, you'll have to patch that ConfigEntries.qml manually.
*** This bug has been marked as a duplicate of bug 393630 ***