dash: Fixes wrong separator position
The separator position takes an active drag placeholder into account, but not the placeholders and app icons that are currently animating out. As a result, we may end up with the wrong position and some app icons appear on the wrong side of the separator.
The merge request !1804 (merged) only partially resolves this issue. This is a better fix.
Fixes #3966 (closed), #3799 (closed)