Commit 12845f0eefa reset translationX to 0 to fix the slide-in
animation of the dash when dragging a search result, but 0 is the
wrong value for the thumbnail slider - the next time it will be
shown, it will just pop up instead of sliding in due to translationX
already being at its target value.
Fix this by making _updateTranslation take the actual visibility into
account and call that to set the correct translation in dragBegin.
38c70e73b9