If screen is locked while kwin window highlighting effect is active, it becomes almost unusable — lockscreen (plasma-overlay) isn't visible (can enter the password to unlock), still user environment is on screen.

Reproducible: Always

Steps to Reproduce:
1. Activate highlight kwin effect, moving mouse over taskbar (I'm using icon-tasks if this is important) opened window button. That window would be highlighted.
2. Lock the screen. I have autolocking feature on, so this happens to me often. But Ctrl+Alt+L work well too.
Actual Results:  
Lock screen isn't visible, still highlighted window is on screen. Password could be entered (esc, type password, enter — screen becomes unocked), but you can't see what is going on.

Expected Results:  
Plasma-overlay should be seen after locking the screen.

Killing kwin from tty helps to show lock screen. I think this is major bug due to lock screen doesn't perform one of its security features — hide user content from the screen.