It would be nice if auto-hidden panels could have a slide animation with a configurable speed. This was available in plasma 4. Currently the appearance/disappearance is instant. Reproducible: Always
Looks like a KWin thing, the _KDE_NET_WM_SCREEN_EDGE_SHOW that apparently causes the panel to hide doesn't animate its disappearance, although Plasma sets the sliding effect.
That's correct, it's not hooked into the effect system.
Fixed with 5ee958ca7e4c37f32b9311124cc2f57126fc602c and c6b25627105a21f2338123f08dd4281bed17c320