The WordPress developers have released version 6.4 of the CMS in question on Nov. 7, 2023. It is a new major release with more than 100 updates to improve performance for a faster and more efficient experience. It's causing issues.
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The release notes for WP 6.4 are available here. Within my blogs I use the theme RCG Forest for mobile devices. The update to WP 6.4 has destroyed that theme – CSS positioning errors. I'm expecting a fix in WP 6.4.2, see here.
Change CSS align-item from start / end to flex-start / flex-end for full browser support
WordPress 6.4.1 released
WordPress 6.4 was released a couple of hours before, but this led to problems. The developers have now released bug fix version 6.4.1, which, according to the release notes, fixes four bugs. However, the bug in the mobile device display here on the blog has not yet been addressed.
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Followed quickly by 6.4.1 to fix some regressions:
https://wordpress.org/documentation/wordpress-version/version-6-4-1/
Thx, I've recognized and mentioned within the German blog. WP 6.4 destroyed my mobile devices theme RCG Forest. The fix probably is scheduled for 6.4.2.
Addendum: I've described a quick fix for the broken mobile designs using a plugin – currently only available in German within this blog post.
They really messed up this time, many 6.4.1 and 6.4.2 fixes are simply rollbacks of changes in 6.4.