fix: Check that added/removed nodes are still present/absent
fix: Check that added/removed nodes are still present/absent

An element that’s moved to a different position in the DOM (such as by
the constructor of a containing SimpleBar) can show up as both removed
and added in a mutation record, so we need to test whether the element
is currently present.

Fixes #552.

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

Grsmto/simplebar,Grsmto/simplebar,Grsmto/simplebar