[charcircuit]

>A software cursor is manually drawn by the graphics stack, which saves what was under it, draws the cursor, and then whenever it needs to be moved, writes the original data back, saves the data at the new position, and then draws the cursor there.

If a hardware layer is not being used the cursor layer will be treated like any other layer in the compositor. Modern compositors don't try and save and write pixels like that. It will just rerender it.

>(which is normally done per mouse interrupt);

It's normally done every frame the compositor makes.

>or it may end up drawing over what software wants to draw

The compositor composites everything at that will be shown on the next refresh of the display. Things don't indepently step on each others toes since it's just the compositor rendering and synchronizing all hardware layers (planes).