Block toolbar removes bottom shadow of header

[t-hamano]

Description

Reported in this comment: #61835 (comment)

With any block selected, if you scroll the canvas up and part of the block toolbar overlaps with the header, the header shadow disappears.

I used git bisect to investigate the issue, and it looks like this issue first appeared in #61835.

Step-by-step reproduction instructions

• Disable the Top Toolbar.
• Click any block.
• Scroll down the canvas.
• When the block is visible in the canvas, the block toolbar will be docked below the header.
• Scroll the canvas some more.
• As the block becomes invisible, the block toolbar will overlap the header.
• The shadow below the overlap should disappear.

Screenshots, screen recording, code snippet

dd91690c4856cdf5bb7d17d19e4f84f9.mp4

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

[stokesman]

I've looked at this a little more as it’s pretty curious. I see that the block toolbar is in front of the header by z-index but doesn’t visibly overlap (besides the shadow) because its overflow is clipped by siblings of the header. Since the shadow isn’t part of the box model the overlap is possible (as opposed to when it was a border).

I also wanted to add that in case we want to consider a fix where we don’t have to fiddle about with z-index, I noticed applying isolation: isolate; on .interface-interface-skeleton__body or .editor-visual-editor solves it (not sure if which is the better place to put it). Of course, it needs more testing to make sure there’s not some unintended effect, but I tested other popovers and they still go in front of the header as expected.

[t-hamano]

This is the first time I've heard of such CSS 😅 From what I've tested so quickly, it does seem to solve the problem.

[stokesman]

Well, I went ahead and made a PR of it ☝️.

Aki, it seems to me this would qualify as a regression since the last release and should be on the 6.6 board no?

[t-hamano]

Aki, it seems to me this would qualify as a regression since the last release and should be on the 6.6 board no?

That's right, I'll add it to the board.