Cancel left margin when full width block appender used at root level of layout

[getdave]

Description

This PR ensures that when a full width block appended is used at the root level of the block list then it doesn't have any left margin applied to it.

It looks like we've been tinkering with the block appended margins quite a bit so it's obviously a thorny area.

This fixes #33085

How has this been tested?

I don't have a high degree of confidence in the test scenarios I'll need to run through but here's a start:

• Activate TT1.
• Go to Appearance -> Widgets
• Look at the block list appender - check there is no additional left margin.
• Also check full width appender that is used in the Group block.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[getdave]

@jasmussen I'd really appreciate your 👀 on this one as I expect there are a number of test scenarios I'll need to run through to ensure this change doesn't introduce any regressions.

[jasmussen]

Thanks for the PR.

I'm still investigating why the margin is there in the first place. It's also present, but cancelled out, in the block editor when inserting a Group block:

I feel like it's there for a reason, and if not, we can remove it entirely. But I'll dig a little deeper and get back to this one!

[jasmussen]

Okay, so here's why the left margin is there:

so it has to have an explicit left margin that isn't "auto", in order to work inside flex containers:

But obviously when the big white plus button is used, that margin is useless.

Let me look at whether there's a better way to cancel out that margin. The thing is, I'd like to remove that in-canvas inserter entirely, in favor of something that's abs-positioned/popovered, for some of the reasons outlined in #31886. So I'd love to find CSS that doesn't assume that black plus will exist forever, instead of overriding it.

[getdave]

We really need a way to distinguish between the "plus" appended and the full width one.

[jasmussen]

Yep, I'm looking into that now. block-list-appender__toggle is only used on the black plus, and block-editor-button-block-appender is the only remaining class on the white one.

I think there's an opportunity for a refactor, also in context of #33025, but let's find a small fix in the mean time. I shall return in a bit.

[jasmussen]

I have #33088 as an alternate fix.

[getdave]

Closing in favour of #33088