Opening the toolbar feeds its height back into scrollTop so rows keep
their position on screen. At the top of the list there are no rows above
to hold steady, so that scroll instead slides the first message under the
toolbar — select the top message and it disappears behind the bar acting
on it.
Skip the compensation when the list is already at the top and let the
content move down there, which is what the toolbar taking up space
should do. Mid-list behaviour is unchanged, and the scroll is clamped at
zero so collapsing the toolbar cannot drive it negative.
Shuki Vaknin
71dd5f4fed