Created attachment 91023 [details]
Screenshot of resize handle visible in smplayer in fullscreen with breeze decoration

When opening a an app in fullscreen, such as fullscreen youtube in a web browser, or smplayer, vlc, etc., the resize handle that's present in the lower right corner of the window can be seen in an arbitrary position on the screen (the position is somehow related to the non-fullscreen window dimensions). Note that border size must be set to "no borders" to reproduce this bug. I have tested Breeze and Plastik themes as these are the only ones that I have on the system.
Additionaly, if one grabs the handle in the fullscreen and attempts to resize window, kwin crashes.

Steps to reproduce:
1. Use Breeze or Plastik decoration themes.
2. Set "no borders" for window decorations. Windows now have a resize handle in the lower right corner.
3. Open an app capable of fullscreen and switch to fullscreen mode. Tested with VLC, Smplayer, mpv, youtube in fullscreen in Firefox, fullscreen browsing in Firefox.

Expected results:
No window resize handle should be visible when using apps such as video players or web browsers in fullscreen.
Observed results:
Window resize handle is visible on the screen during fullscreen. Grabbing it and attempting to resize the window crashes kwin.

I have attached a screenshot of smplayer showing the resize handle "in the middle" of screen and a backtrace from gdb from kwin_x11 crash.