The CSS Grid Page Overlay should correctly represent lines in a masonry layout. The CSS Grid Page Overlay is currently broken because it relies on there being grid tracks on both axes (see screenshot). Technically, there are no grid tracks in the masonry axis. Grid cells vary in dimension in the masonry axis. The masonry layout implementation is currently behind a flag ("CSS Masonry Layout") in Safari > Settings > Feature Flags.
Created attachment 468766 [details] Screenshot of issue
<rdar://117505876>
<rdar://problem/118833618>
rdar://117505876
Pull request: https://github.com/WebKit/WebKit/pull/20934
Created attachment 468767 [details] Screenshot with patch applied
Created attachment 468768 [details] Test case for masonry + writing modes
Committed 271332@main (9a3f6dd4c6a6): <https://commits.webkit.org/271332@main> Reviewed commits have been landed. Closing PR #20934 and removing active labels.