This is strange. What case did you find where this was a problem?
height of a box should never be AUTO when layout is complete.
Looking at this more closely, I think that we need to move the call to
layout_absolute_children() later in layout_block_context(). Currently it
is done before the non-absolute children are laid out, so height can be
AUTO. It needs to be moved to when we go up the tree. I'll have a look at