Replace content_box_rect_in_static_position_ancestor_coordinate_space()
which walked the ancestor chain to compute the offset between an abspos
element's static position containing block and its actual containing
block.
Instead, add a cumulative_offset() method to UsedValues that computes
the absolute offset from the ICB to a box's content edge by walking the
containing block chain. For pre-populated nodes during partial relayout,
it returns a cached value from the paintable's absolute position.
The static-CB-to-actual-CB offset is now a simple subtraction of two
cumulative offsets, which also fixes a bug where table cells with
position:relative ancestors got incorrect static positions due to the
old function accumulating offsets in the wrong coordinate space.
Also route direct UsedValues::offset assignments in Flex, Grid, and
Table formatting contexts through set_content_offset().
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.
This occured because the abspos containing block is in-between the
containing blocks of two regular ancestors in this case, so it was
being skipped. (The containing block of a table cell is the table
itself, not the table row.)
