Bug 2038072 added the support to allow abspos frames, whose containing block is
an inline frame, to live under a later inline continuation. This bug cannot
happen when `layout.abspos.fragment-aware-inline-cb.enabled = false`.
In an incremental reflow scenario, an inline frame can already be split into
several continuations. However, if an inline frame becomes complete (e.g. during
a measuring reflow), we want to pull the absolute children's first-in-flows from
the inline frame's next-in-flows' absolute child lists before those
next-in-flows get destroyed. Otherwise, there might be dangling placeholders
without out-of-flows.
Differential Revision: https://phabricator.services.mozilla.com/D311575
Ting-Yu Lin
8b966872e2