When a popup is placed using move_to_rect(), it'll get feedback about
the position and size it got assigned. We use this feedback to update
the scroll offset, but while doing so, if the visibility of the arrow
changed, we didn't adapt the offset accordingly.

Fix this by offsetting the provided offset by the height of the arrow,
if it was made visible as a side effect of the scroll offset change
triggered by the feedback.

Related: mutter#105
Closes: #1463