If you have a panel with its visibility set to Auto Hide, the desktop effect making the panel slide in and out of the screen edge will not work the first time the panel is presented, after desktop effects have been disabled and enabled on the run. Instead the panel will first appear and possibly disappear instantly, then the gliding effect will start working again.

To test this: Make sure the panel is set to auto-hide. Whenever you place the pointer over the screen edge, you should see it smoothly slide out of that edge... once you take the pointer off of the panel, it should slowly pop back in. Now press Alt + Shift + F12 twice to disable then re-enable desktop compositing. Then bring your mouse pointer to the screen edge once more: This time the panel will appear instantly, the sliding effect will not be present. It's only after the panel is hidden again and you once more place the mouse cursor over the edge to make it show, that the transition effect kicks back into action and works as intended.

This is a minor visual glitch, but since it can be obvious I considered it worth reporting. It has been present ever since the Plasma panel supported smooth transitions for auto-hide via desktop effects. I assume it has to do with the effect not being re-enabled immediately after desktop compositing is, but instead after the panel is first shown after compositing is back on.