Fixed top toolbar: Bottom border missing in the Site Editor

[t-hamano]

Description

In the Site Editor, when "the top toolbar" feature is activated, the bottom border disappears. This problem does not occur in the submission editor. Nor does it occur when the Gutenberg plugin is disabled.

After a little digging, I found that this bottom border is applied to the header (.edit-site-header-edit-mode) with a height of 40px. The border would disappear because a toolbar with the same 40px height and white background would overlap on top of it.

Step-by-step reproduction instructions

N/A

Screenshots, screen recording, code snippet

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[ramonjd]

I got something up here: #54131

Not really sure if it's the best fix, but we're a bit constrained by the hard-coded 60px height of the header.

Also noticed the following in all browsers on trunk:

2023-09-03.12.30.00.mp4

[ramonjd]

Resolved by #54356