Since bug 251830, the wide-layout attachment still uses legacy attachment code to display the progress indicator. This should be reimplemented inside the new shadow DOM tree, to reduce the dependency on legacy code.
<rdar://problem/107716507>
Pull request: https://github.com/WebKit/WebKit/pull/12424
Committed 262758@main (31c3e79e949e): <https://commits.webkit.org/262758@main> Reviewed commits have been landed. Closing PR #12424 and removing active labels.