Version:            (using Devel)
Installed from:    Compiled sources

First: this does not happen with horizontal panels at all (I've tried).

When e.g. the Kmenu or the device notifier hides the animation stops before the entire "thing" (menu or whatever) is hidden. It is not "really there" anymore, as clicking in it will cause a redraw of the underlying application and the debries are mostly removed. If I start an application and then just go with the jumping icon on the mouse pointer over this area the tracks of the icon will then show the application below this.

The width of this "debries" is the same as the width of the panel itself. Looks like the animation is intended to stop when the right corner (for a left vertical panel) is below the right edge of the panel but the right edge is calculated starting at the right edge of the panel and not the left edge of the screen.

Sooo much words. Just test it, you will probably see it.