Clicking the right mouse button, the context menu only appears after moving the mouse pointer; this seems to be an issue when the "fading popups" effect is disabled.
What are you right-clicking on? I ask because I right-clicked on a bunch of things and couldn't make this happen. And are you using X11 or Wayland? And with what distro?
X11, OpenSuse TumbleWeed. Right clicking anywhere, Kate, Dolphin, Konsole. It also seems to affect the menus in the menu bar. Setting the animation speed in the general behaviour KCM to a very low value seems to "fix it".
Thanks. Weird. No idea what could be causing this, and I kind of doubt it's a KWin issue. More likely to be something in Qt, I suspect.