Make nsIFrame::PeekOffsetForLine won't cross editing host boundary unless the callers allows that
The problems in the testcase are, nsIFrame::PeekOffsetForLine returns a frame
for any content node outside the editing host and nsIFrame::GetLastLeaf
returns a native anonymous subtree node if the frame is for the native
anonymous subtree root like <input> or <textarea>.
For fixing the former, the methods need to check whether found frame's editable
state and whether the editable node is an inclusive descendant of the
editing host. However, there are complicated cases with inline editing hosts
and elements whose contenteditable is set to false and this is a new
regression for ESR 128. Therefore, we need to fix this without any behavior
changes as far as possible. Therefore, this patch basically checks only whether
the editing state of found one is editable and/or whether the one is an
inclusive descendant of the editing host to avoid Selection moves outside the
editing host. The reaming complicated cases should be handled in new bugs which
blocks bug 1873155.
For fixing the latter, nsIFrame::GetLastLeaf needs to check the given frame's
content is a native anonymous subtree root or not.
Differential Revision: https://phabricator.services.mozilla.com/D216371
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1897255
gecko-commit: e90eb44c2da3303c038c67b7ad28b1498cd5a98c
gecko-reviewers: emilio