WebKit export of https://bugs.webkit.org/show_bug.cgi?id=321874 - #64173

WebKit export from bug: Accessible name of a display: contents element includes text from its siblings

Accessible name of a display: contents element includes text from its siblings

Name from contents walks the element's children by taking its first child and then repeatedly the next
sibling. A display: contents element has no box for its children to hang off, so the object after its last
child is one of the element's own siblings, and the walk carries on into it. The check meant to stop this only
did so when the next object had a display: contents parent of its own that differed, which the siblings do
not have, so nothing stopped it. Ask instead whether the next object is still a child of the element we
started from.

Chrome and Firefox name all of the cases in the test "x". Whether the leak was visible depended on what
followed in the markup, which is why the div at line 66 of
css/css-display/accessibility/display-contents-role-and-label.html reported "x x x x" while the same shape
with the link role a hundred lines later reported "x": the latter is followed by a display: contents list
whose item did trip the old check, and the two objects it had already walked into were whitespace only.

WebKit-Bug: https://bugs.webkit.org/show_bug.cgi?id=321874
WebKit-Canonical-Link: https://commits.webkit.org/319287@main