Navigate regions focus style bug with Safari

[afercia]

Description

Follow-up to #45369

Turns out the Navigate regions focus style isn't visible on the content area with Safari, under certain conditions. This seems to be a Safari bug, which is known to have problems with z-indexes.

The bug can be reproduced only when the post has some long content.

Step-by-step reproduction instructions

Test with Safari:

• Edit a post with some short content, e.g. a couple one-line paragraphs.
• Press Control+backtick to navigate through the editor regions.
• Observe the focus style (a blue outline) is visible when focusing the content area, as expected. See the animated GIF below:

• Edit a post with some long content that overflows the viewport.
• Press Control+backtick to navigate through the editor regions.
• Observe the focus style on the content area is not visible. See the animated GIF below:

• While the content area is focused, hover the mouse between two blocks to make the 'in between inserter' appear.
• Observe the focus style becomes visible when the in between inserter appears. See the animated GIF below:

Screenshots, screen recording, code snippet

No response

Environment info

• macOS Monterey 12.6
• Safari 16.0 (17614.1.25.9.10, 17614)

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

[afercia]

Can't reproduce any longer after recent changes. Closing.

Note: to make sure this kind of visual regressions don't happen again in the future, having some visual regression tests would be great. #46210 is a proof of concept in that direction. However, introducing visual regression tests requires some higher-level discussion to establish a proper workflow.