TextLeafPoint::FindBoundary needs to support BOUNDARY_PARAGRAPH. This should be fairly similar to the paragraph code in HyperTextAccessible, since it already crawls the a11y tree (it doesn't use layout).

This information is already available via Attributes, but we don't want to retrieve all Attributes just to get the display style.

This is achieved via a new FILTER_WALK_SUBTREE_IF_CONTAINS_ANCHOR which is combined with FILTER_IGNORE_SUBTREE.
This is necessary when searching for a paragraph boundary, since we want to find a containing block but we don't want to descend into other blocks.

Comment on attachment 9263885 [details]
Bug 1730090 alternative part 2: Add Accessible::IsBefore/IsAncestorOf.

Revision D138743 was moved to bug 1755604. Setting attachment 9263885 [details] to obsolete.

Backed out 4 changesets (bug 1730090, bug 1755604, bug 1755420) for causing build bustages in accessible/base/TextLeafRange.cpp

Backout link: https://hg.mozilla.org/integration/autoland/rev/6938c3b26d145c7e2c77f078566463df66d45359

Push with failures

Failure log

NFO -  In file included from Unified_cpp_accessible_base1.cpp:29:
[task 2022-02-18T06:06:16.637Z] 06:06:16    ERROR -  /builds/worker/checkouts/gecko/accessible/base/TextLeafRange.cpp:361:15: error: expected unqualified-id
[task 2022-02-18T06:06:16.637Z] 06:06:16     INFO -      if (RefPtr{aAcc->DisplayStyle()} == nsGkAtoms::block ||

https://hg.mozilla.org/mozilla-central/rev/a59c38e062bd