How to "keep above others" window at slide show?

So far I fail to keep another window above a full screen slide presentation. I have set the other window to “More Actions”->“Keep Above Others”. Though, when switching to presentation mode, that window goes away. I have tried presentation in a non-full screen presentation, but then other items show up on the screen; it is not just showing the slides.

Purpose: I want to show Kcalc above the presentation window, and keep it there. At the moment, despite my settings, I have to fetch that window of Kcalc, do my maths, but at any click to advance the presentation, the window containing Kcalc disappears.
How can I actually tell the system to keep another window above presentation mode?

Thanks in advance,

Uwe