Disabled tree shaking in Customizer causes stylesheet exclusions

[westonruter]

Bug Description

I discovered a regression when testing Twenty Fifteen as a Reader theme. When I opened the AMP Customizer, I saw the theme without any styling:

I see the same thing if I view the changeset preview link (https://wordpress-stable.lndo.site/hello-world/?amp=1&customize_changeset_uuid=9c4fa0b0-3e3a-48a3-92a6-90b2812802e6)

If I validate that changeset preview I see the stylesheets as follows:

Note that two are being excluded. No tree shaking is being performed:

It is expected that tree-shaking is disabled in the Customizer preview, as Customizer scripts required dynamic style rules. What is not expected, however, is that stylesheets are getting excluded.

I can also reproduce this issue in Twenty-Twenty One and Twenty Twenty.

It does not happen in Standard mode because excessive CSS is kept by default in Standard mode.

Expected Behaviour

Excessive CSS should be kept by default in the Customizer preview.

Screenshots

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PHP Version

No response

Plugin Version

2.2

AMP plugin template mode

Transitional, Reader

WordPress Version

No response

Site Health

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Gutenberg Version

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OS(s) Affected

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Browser(s) Affected

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Device(s) Affected

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Acceptance Criteria

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Implementation Brief

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QA Testing Instructions

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Demo

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Changelog Entry

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[westonruter]

The regression was introduced in #6546 in which the allow_excessive_css arg was added to AMP_Style_Sanitizer to be a more explicit opt-in to keeping excessive CSS, instead of using skip_tree_shaking to imply that excessive CSS should be kept:

So the only thing we need to do is:

--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -1560,7 +1560,8 @@ function amp_get_content_sanitizers( $post = null ) {
 		],
 		AMP_Block_Sanitizer::class             => [], // Note: Block sanitizer must come after embed / media sanitizers since its logic is using the already sanitized content.
 		AMP_Style_Sanitizer::class             => [
-			'skip_tree_shaking' => is_customize_preview(),
+			'skip_tree_shaking'   => is_customize_preview(),
+			'allow_excessive_css' => is_customize_preview(),
 		],
 		AMP_Meta_Sanitizer::class              => [],
 		AMP_Layout_Sanitizer::class            => [],

[westonruter]

QA Passed per #6788 (comment)