Compiz used to have this neat effect that allowed you to see the windows below the active window by making the active window transparent when the mouse was moved outside it. This is similar to the Translucency desktop effect, but instead of having to move the active window, it was based on the mouse focus.
This is not the same as focus-follows-the-mouse, where the inactive window is raised above the active one. The compiz effect was just changing opacity to 10% when the active window was losing focus and back to 100% when the mouse was again over the active window.
Is it possible to re-create this effect with Plasma?