layout: Simplify `PositioningContext`

`PositioiningContext` held two vectors, one inside an `Optional`, to
differeniate between the version used for a containing block for all
descendants (including `position: absolute` and `position: fixed`) or
only for `position: absolute` descendants. This distinction was really
hard to reason about and required a lot of bookkeeping about what kind
of `PositioningContext` a layout box's parent expected. In addition, it
led to a lot of mistakes.

This change simplifies things so that `PositioningContext` only holds a
single vector. When it comes time lay out hoisted absolutely positioned
fragments, the code then either:
 - lays out all of them (in the case of a `PositioningContext` for all
   descendants), or
 - only lays out the `position: absolute` descendants and preserves the
   `position: fixed` descendants (in the case the `PositioningContext`
   is only for `position: absolute`.)

It's possible that this way of dealing with hoisted absolutes is a bit
less efficient, but in general, the number of these descendants is quite
small, so it should not be significant. In addition, this decreases the
size in memory of all `PositioningContexts` which are created in more
situations everyday.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

0 files changed, 0 insertions, 0 deletions