Empty toolbar element is visible when editing template parts in isolation

[carolinan]

Description

When you create a new template part or edit a template part in isolation, and reduce the browser window width,
there is an empty element with the class ".block-editor-block-contextual-toolbar" at the top of the editable area/ canvas, inside the dark grey border.

Because it is inside the dark grey border, it looks like it is in the editable area and the elements visible border is confusing.

In comparison, when editing a template in isolation in the template editor, this element is above the dark grey border.
When editing a template in the Site Editor, the element is above the canvas.

Step-by-step reproduction instructions

1. Activate a block theme
2. Go to the Site Editor and select a template part and choose "Edit". Or, create a new template part.
3. Once in the template editor, reduce the browser window width.
4. See that there is a border showing at the top of the editable area.

(In Twenty Twenty-two, using a dark header template part, it will show as a white field above the dark header)

Screenshots, screen recording, code snippet

Template part:

Template in template editor:

Template in Site Editor:

Environment info

6.0 RC 2.
With and without Gutenberg trunk
beta tester plugin

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

Yes

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

No

[carolinan]

Perhaps instead of having the bottom border on block-editor-block-contextual-toolbar, perhaps on this screen, it could be added to the inner element that is only present on the page when the toolbar is populated?

[ramonjd]

This is even more obvious now in the site editor "view" canvas mode:

only present on the page when the toolbar is populated?

What about hiding it where no blocks are selected? Trying that out here: #46298

[carolinan]

@ramonjd If I understood the discussions in the PR correctly it was a partial fix, should this issue be reopened?

[ramonjd]

Thanks @carolinan Yeah it ended up targeting the site editor view mode only. I'll reopen.

Maybe #40450 will go some way to address this issue

[carolinan]

I can not reproduce this in 6.3 RC1.