I started noticing an issue with the popup-menu that displays the list of documents when you press Ctrl+Tab. It will render behind the main window, so I can no longer see it.
If I move the window to another screen, the popup still renders on the previous location so it is now visible (as the main window is no longer in that location).

I think this started to happen around the time I updated to Qt Creator 13, but I cannot rule out that this is a Plasma bug. It's not apparent form the start of the program, but enters this state after a while.
If I move the main window to another screen I can click the context menu with the mouse and it will try to resize the main window. After doing this the document list context menu is back to normal, but if I switch focus to another window and back to Qt Creator, it breaks again.

I've tried to run Qt Creator in XWayland, and it appears that this is not an issue there, but due to Nvidia sync issues causing flickering, I have not done extensive testing as it makes the application hard to use.

I'm currently running Arch Linux with KDE Plasma 6.0.4 and KWin 6.0.4.1 on Nvidia 550.76-1 with open GPU kernel modules.