Remove initial-scale=1 meta viewport property by default to disable tap delay

[westonruter]

Summary

Fixes #5894

Behavior can be disabled via this plugin code:

add_filter(
	'amp_content_sanitizers',
	function ( $sanitizers ) {
		$sanitizers[ AMP_Meta_Sanitizer::class ]['remove_initial_scale_viewport_property'] = false;
		return $sanitizers;
	}
);

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

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[codecov]

Codecov Report

Merging #5903 (429ff34) into develop (ced3165) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@              Coverage Diff              @@
##             develop    #5903      +/-   ##
=============================================
- Coverage      75.12%   75.12%   -0.01%     
- Complexity      5667     5670       +3     
=============================================
  Files            210      210              
  Lines          17023    17027       +4     
=============================================
+ Hits           12789    12792       +3     
- Misses          4234     4235       +1     

Flag	Coverage Δ	Complexity Δ
javascript	75.13% <ø> (ø)	0.00 <ø> (ø)
php	75.12% <75.00%> (-0.01%)	0.00 <0.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
includes/sanitizers/class-amp-meta-sanitizer.php	79.79% <75.00%> (-0.21%)	35.00 <0.00> (+3.00)	⬇️

[pierlon]

Will this resolve the page experience guide check? According to the linter rule, if the content value of the meta[name=viewport] property doesn't exactly equal width=device-width, the test will fail.

[westonruter]

It should resolve it at least for the core themes which have width and (often) initial-scale, right?

[pierlon]

Right, in the context of the core themes this would be fine.

[pierlon]

This looks good. Themes that make use of other viewport properties (such as maximum-scale=1) will not benefit from this even when the content value is reduced to width=device-width, although I would assume such occurrences to be rare.

[westonruter]

Yeah, for other themes then the PX check would complain and that's good. If a theme is using other viewport properties we can't safely automatically remove them as we can apparently do for initial-scale=1. For them the theme authors should fix the viewport, at least for now. Maybe we'll go further in the future.