According to wktesthunter this started to happen between [r251717, r251734] $ git log --oneline $(gitrev r251717)..$(gitrev r251734) 81c9f3b4433 [iOS] Clean up sandbox to group similar rules together https://bugs.webkit.org/show_bug.cgi?id=203525 <rdar://problem/56686416> 513a1dc188e imported/w3c/web-platform-tests/css/css-sizing/dynamic-available-size-iframe.html is flakey https://bugs.webkit.org/show_bug.cgi?id=203517 fba458ccfb7 REGRESSION (r251413): Nightly build crashes on launch due to -[WKPreferences _setShouldAllowDesignSystemUIFonts:]: unrecognized selector https://bugs.webkit.org/show_bug.cgi?id=203549 9031fdf2253 Flaky Test: imported/w3c/web-platform-tests/css/css-values/absolute_length_units.html https://bugs.webkit.org/show_bug.cgi?id=203581 87599843af9 css/css-sizing/aspect-ratio-affects-container-width-when-height-changes.html is flakey https://bugs.webkit.org/show_bug.cgi?id=203583 54ecc2704db Update css/css-multicol WPT https://bugs.webkit.org/show_bug.cgi?id=203551 043aa74a91a [ANGLE] Window Server blocking stops single GPU systems from creating a CGLPixelFormat https://bugs.webkit.org/show_bug.cgi?id=203522 <rdar://problem/56685302> eca03ed6711 Add enterkeyhint support https://bugs.webkit.org/show_bug.cgi?id=189546 <rdar://problem/51021148> 2a2130eeff4 Layout Test http/tests/resourceLoadStatistics/website-data-removal-for-site-navigated-to-with-link-decoration-database.html is a flaky failure https://bugs.webkit.org/show_bug.cgi?id=203542#add_comment <rdar://problem/56689764> 0c8daf52382 <input type="range">.setAttribute("value") does not update the value https://bugs.webkit.org/show_bug.cgi?id=202859 <rdar://problem/56204271> I bet it was 54ecc2704db aka r251728 Update css/css-multicol WPT https://bugs.webkit.org/show_bug.cgi?id=203551 The failure doesn't happen on Mac. It can be reproduced on WPT: https://wpt.fyi/results/css/css-multicol/multicol-gap-percentage-001.html?label=master&label=experimental&product=chrome&product=firefox&product=safari&product=webkitgtk&aligned