Previously, has_scrollable_overflow was a purely geometric check, true
whenever content extended beyond the padding box regardless of the
overflow property. This caused unnecessary scroll frame allocation for
boxes with `overflow:visible`.
Per CSS Overflow 3, scrollable overflow is only defined for scroll
containers (overflow: auto/hidden/scroll). Gate the flag on
`is_scroll_container()` so that only actual scroll containers get scroll
frames assigned.
Inline nodes in our layout tree have a position, so let's show it. By
centralizing the logic for this, block nodes now lose their redundant
'content-size' dump info which is already part of the box model dump.
...with inline children. This fixes an issue when we ignore abspos boxes
contained by PaintableWithLines while calculating overflow rect size.
Lots of layout tests are affected, because now PaintableWithLines has
overflow rect.
`Text/input/DOM/Element-set-scroll-left.html` is also affected and now
matches other browsers.
This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
When there are floats present inside an IFC, we must coordinate with
the parent BFC to calculate the automatic width of the IFC's block box.
This is because the IFC is not directly aware of floats. Only the BFC
knows enough about them to account for them in automatic sizing.
