wp-build: Fix black flash on wp-admin pages before hydration

[itzmekhokan]

What?

Fixes a brief black flash when navigating to wp-admin integrated boot pages (such as the Connectors screen) before React hydrates.

Why?

The page-wp-admin.php.template critical CSS set #wpwrap background to var(--wpds-color-fg-content-neutral, #1e1e1e). That is a foreground design token used as a background, so before @wordpress/boot hydrates and applies .boot-layout styles, the content area briefly renders as near-black.

This matches the boot layout background used after hydration:

.boot-layout {
	background: var(--wpds-color-bg-surface-neutral-weak, #f0f0f0);
}

How?

Updates #wpwrap critical CSS in packages/wp-build/templates/page-wp-admin.php.template to use var(--wpds-color-bg-surface-neutral-weak, #f0f0f0).

Generated page-wp-admin.php files in WordPress Core are built from this template, so the fix must land here rather than by editing generated files directly in wordpress-develop (see Core Trac #65247 and @westonruter's feedback on wordpress-develop PR #11892).

Testing Instructions

1. Build/sync Gutenberg into WordPress Core (or use the Gutenberg plugin with a wp-build output that includes this template change).
2. Navigate to Settings → Connectors in wp-admin.
3. Confirm there is no black flash before the interface renders.
4. Confirm the initial background matches the boot layout gray (#f0f0f0) before and after hydration.

Related

• WordPress Core Trac: https://core.trac.wordpress.org/ticket/65247
• Prior wordpress-develop PR (closed per review): Administration: Fix black flash on Connectors screen before hydration. wordpress-develop#11892

[github-actions]

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Co-authored-by: itzmekhokan <khokansardar@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>

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[westonruter]

Note: The old value was originally introduced in c69eedf as part of #73977.

[westonruter]

There is no black background color flashing after this change, so that's good.

Before:

Screen.Recording.2026-05-21.at.23.14.11.mov

After:

Screen.Recording.2026-05-21.at.23.13.32.mov

[t-hamano]

This matches the boot layout background used after hydration:

.boot-layout {
	background: var(--wpds-color-bg-surface-neutral-weak, #f0f0f0);
}

Is this true? Isn't the actual background of the rendered page --wpds-color-bg-surface-neutral?

Also, as another approach, I'm curious what happens if we remove the background color itself from #wprap, since the body element already has a white background applied.

[itzmekhokan]

This matches the boot layout background used after hydration:

.boot-layout {
	background: var(--wpds-color-bg-surface-neutral-weak, #f0f0f0);
}

Is this true? Isn't the actual background of the rendered page --wpds-color-bg-surface-neutral?

Also, as another approach, I'm curious what happens if we remove the background color itself from `#wprap`, since [the body element already has a white background applied](https://github.com/WordPress/gutenberg/blob/67c44e05fc2be4800cca4459b62474b2241a78ce/packages/wp-build/templates/page-wp-admin.php.template#L266-L268).

@t-hamano Thanks for the thoughtful review — both points are valid.

You're right that the visible Connectors content area uses --wpds-color-bg-surface-neutral on .boot-layout__canvas / __stage, not --wpds-color-bg-surface-neutral-weak on the outer .boot-layout shell. My earlier description was imprecise.

I also agree that removing the #wpwrap background is the cleaner fix. Looking at #73977, the #wpwrap rule was moved into critical CSS mainly for overflow-y: auto (the high-zoom admin menu scroll fix). The background: var(--wpds-color-fg-content-neutral, #1e1e1e) was a foreground token mistakenly applied as a background, which caused the black flash.

I've updated the PR to:

Remove the #wpwrap background entirely
Keep overflow-y: auto for the #73977 scroll behavior
Rely on the existing body { background: #fff; } for pre-hydration paint, which better matches the canvas surface

Could you take another look when you have a moment?

[t-hamano]

LGTM! Finally, could you just resolve the code conflicts?

[simison]

FYI, adding missing changelog entry for this change in #78807