I use the KDE plasma desktop.
For my teaching, I would like to have the ability to have the mouse leave a trail for a few second. The idea is that the trail fades away after a predefined time. This would allow me to draw arrows, or circle certain areas. I would like this functionality at the desktop level, not at the level of an individual window.
Since I have not been able to find such an effect, I consider writing my own. The idea would basically be to place a transparent layer on top of the desktop, and follow mouse movements. GUI events would be forwarded to the underlying layers, except for a key sequence to activate/deactivate the mouse trail.
For the moment, my question is where such an effect could best be "hooked into the desktop". I know there are "Desktop effects", but maybe there also is the possibility to write a simple KDE/Qt application that has a transparent window that always sits on top.
Reading the question it looks like more of a screen drawing thing than mouse trail... This may not be exactly what you are looking for, but I always use Gromit-mpx for presentations. It allows drawing on top of anything on screen just need to learn some shortcuts (enable/disable/earase/clear). Or just map them as express keys on a drawing tablet.