Improve scrollable element detection when zoomed

Improve scrollable element detection when zoomed

Elements may overflow when zooming in or out. However, the `.scrollHeight` of
the element is not correctly updated when the 'overflow' event occurs, making it
possible for unscrollable elements to slip in. An example is Google Groups.
There, the entire page gets an 'overflow' event when zooming, and by then it
actually looks to be scrollable. When inspecting the element afterwards, though,
it becomes clear that it is not. Well, actually, then it looks like the element
is scrollable by 1px. So this commit also sets a minimum scroll to 5px.

The solution to the zoom problem is to check if the largest scrollable element
really is scrollable before trying to scroll it. If not, update it.

See #605.