Originally I thought we'd benefit from this change in this branch; turns
out that we didn't after all, because we changed the approach, but it's
a nice cleanup anyway, so we include it here.
Route all mouse events to the view that was under the pointer when the
left button was pressed, until the button is released. Previously each
event went to whatever view was under the pointer at the time, so a
drag that left the view's bounds started acting on neighboring views.
Since events can now carry positions outside the view, clamp the view
cursor to the view's bounds in that case (handlers still receive the
unclamped position), and require an actual click for tab activation so
that a captured drag crossing the tab row doesn't switch tabs.
