Zoom effect related?

Otherwise there's no chance than kwin hides the cursor - any client however may at any time, eg. konsole and videoplayers typically do after a short while.

> It comes back by itself after a while.
This does not sound like a bug (in kwin) to me then - we'd either break on the counter (in the zoom effect) or not.

(In reply to Thomas Lübking from comment #1)
> Zoom effect related?

I haven't enabled any such effects, nor do I see any "zoom effect".

> Otherwise there's no chance than kwin hides the cursor - any client however
> may at any time, eg. konsole and videoplayers typically do after a short
> while.

But I thought that if a window does something to the mouse pointer, other windows should still have a normal mouse pointer when I hover over them. That's not what I see here. This is not full screen and I have 2 displays with different windows on each, and it happens on both displays, including on the plasmashell panel.

> other windows should still have a normal mouse pointer when I hover over them.

yes, that should work. If it doesn't it just indicates more that this is a problem outside of KWin. E.g. X or more likely drivers.

> But I thought that if a window does something to the mouse pointer, other windows should still have a normal mouse pointer when I hover over them.

No, there're several (ok, "two" common) ways to hide the cursor completely. Any client can do this.
Prove: unclutter.

The zoom effect (kcmshell5 kwineffects) uses one to hide the cursor and show a (scaled) texture instead on zooming the screen - but if that's not the cause, it's *very* most unlikely kwin.

=> Try to figure a pattern (running, active, changing windows) when this happens (forth and back)

As Martin pointed out, this also can be a bug in the X11 driver (the cursor is special and painted by the driver into the scanout buffer after the actual rendering)

You may attach /var/log/Xorg.0.log for inspection.

KWin is not responsible for rendering the cursor on X11.