Sibling Inserter: Broken positioning on Site Editor

[jasmussen]

The sibling inserter shows up when you hover between two blocks, whether in the post or site editor:

This still works fine on the post editor, but regressed on the site editor, where it's offset about 60px upwards:

The precise amount of vertical offset does not appear to be constant, and appears to depend on which blocks are present on the page. With a few cover and spacer blocks, it's very offset. With just paragraphs, it's almost correct.

In trying to figure out how this regressed, I walked back in time on the commits page (https://github.com/WordPress/gutenberg/commits/master?before=e843e5484e2087d1ef546564c09254cc60dc5170+230&branch=master), and from this digging (I really need to learn to bisect), this appears to have been caused by #27860.

[youknowriad]

Is it the same as ##28457 ?

[jasmussen]

It is. My apologies!

[youknowriad]

No worries, just saw it mentioned in a potential PR that can fix it.