The mouse proximity effect that the panels display when they are hidden, causes the mouse to exit the current window which in turn breaks edge panning in applications that have that (since they can never detect the mouse being close to the edge of the window).
In plasma4 disabling desktop effects would also disable this but that doesn't seem to be the case with plasma5.
The problem is really easy to observe as the cursor will even chance to the configured cursor theme when on that "zone", which is easily observable when playing games that customize the cursor.

Reproducible: Always

Steps to Reproduce:
1. Make a panel hiddable
2. Use an application with edge panning, such as a game like Dota 2 (sorry I can't think of another one right now)
3. Try edge panning with composition enabled or disabled.

Actual Results:  
Mouse escapes window which blocks functionality of the window owner

Expected Results:  
Mouse shouldn't leave the application's window when the panel is not shown.