Snackbar: CSS fix for positioning in large screens

[im3dabasia]

What?

Closes #75350

Fixes the success snackbar positioning on the Fonts screen so it appears above the sidebar instead of behind it when saving via keyboard shortcut.

Why?

When saving font changes using ⌘S / Ctrl+S, the success snackbar is rendered behind the sidebar, making it partially or fully hidden.

How?

Adjusts the snackbar positioning so it is correctly visible on bigger viewports, ensuring it is not overlapped by the sidebar.

Testing Instructions for Keyboard

1. Go to Appearance → Fonts
2. Change any font setting
3. Press ⌘S (macOS) or Ctrl+S (Windows)
4. Confirm the modal
5. Verify the success snackbar is fully visible and not hidden behind the sidebar

Screenshots or screencast

Screencast

Screen.Recording.2026-02-10.at.11.37.49.AM.mov

Screenshots

Before	After

[github-actions]

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Co-authored-by: im3dabasia <im3dabasia1@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: juanfra <juanfra@git.wordpress.org>

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[juanfra]

Thanks for the PR @im3dabasia! The fix does solve the visible issue, but I think it's working around the problem rather than addressing the root cause.

The snackbar uses position: fixed, so it's positioned relative to the viewport (not the .boot-layout container). Adding right: 0 only on small screens avoids the overlap with the admin sidebar on larger viewports, but it also means the snackbar still doesn't have proper horizontal bounds above that breakpoint.

The pattern we use elsewhere is to set right: 0 unconditionally and then use the editor-left mixin to offset left based on the admin sidebar state. It'd look something like this:

.boot-layout .components-editor-notices__snackbar {
	position: fixed;
	right: 0;
	bottom: 16px;
	padding-left: 16px;
	padding-right: 16px;
}

@include editor-left(".boot-layout .components-editor-notices__snackbar");

[juanfra]

Closing as the snackbar is center aligned now after #75294.