If there is a remaining margin-bottom in margin collapsing state
tracker after laying out all boxes in the current BFC, it must be
assigned to the last in-flow child since margin collapsing cannot
occur across a formatting context boundary.
The current issue where margin-bottom may be counted twice due to
"collapse through" margins in the last in-flow child box is addressed
with this fix by excluding such boxes during the search for a box to
assign the remaining margin.
Test case coming with this fix has a layout bug with incorrectly
computed line height.
When calculating the intrinsic width of a block-level box, we now ignore
the preferred width entirely, and not just when the preferred width
should be treated as auto.
The condition for this was both confused and wrong, as it looked at the
available width around the box, but didn't check for a width constraint
on the box itself.
Just because the available width has an intrinsic sizing constraint
doesn't mean that the box is undergoing intrinsic sizing. It could also
be the box's containing block!