Fallback to AMP Legacy Reader theme if active Reader theme is unavailable

[pierlon]

Summary

• Obtains current Reader theme data from list of installed WP themes if the supplied Reader theme data becomes unavailable.
• Adds default mobile and desktop image placeholder for themes without a screenshot.
  • Fixes how the desktop screenshot is shown when displayed in the mobile phone container.
• Falls back to the legacy Reader theme if the current Reader theme becomes unavailable.
  • Shows an admin warning notice when in this scenario.

Screenshots:

• Desktop and mobile image placeholders

• Admin warning notice when the AMP legacy theme is used as a fallback, on the Themes and AMP Settings screens:

Fixes #5070
Fixes #5136

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• 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]

Plugin builds for 810ec5f are ready 🛎️!

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[westonruter]

Investigating the test failures…

[pierlon]

Already fixed the E2E ones; pushing fix now.

[westonruter]

@pierlon There's a condition here that may not be accounted for. If I have a plugin present that adding Neve via amp_reader_themes filter, and I select Neve as the Reader theme, but then I delete neve theme, I get an unexpected state where Neve is still the selected theme even though it is not installed:

The only way for me to proceed I believe is to select a non-Neve theme and then re-select Neve, which isn't expected. In the case of Neve still being among the selectable Reader themes but not being installed anymore, the Legacy theme should be selected in that case as well, I suppose?

[pierlon]

Good catch.

In the case of Neve still being among the selectable Reader themes but not being installed anymore, the Legacy theme should be selected in that case as well, I suppose?

Yep that's right. Currently it's only checking if the theme data exists, but it should be instead checking if the theme exists.

[pierlon]

@westonruter e5e086f should fix the issue.

[westonruter]

It doesn't appear that this does anything anymore:

Suggested change

delete_site_transient( 'theme_roots' );

[westonruter]

🎉

[pierlon]

I received the following warning while running the JS tests:

jest-haste-map: duplicate manual mock found: index
  The following files share their name; please delete one of them:
    * <rootDir>/assets/src/components/options-context-provider/__mocks__/index.js
    * <rootDir>/assets/src/components/reader-themes-context-provider/__mocks__/index.js

It seems Jest doesn't allow mocks with the same base name for some weird reason. It's been filed as a bug for quite some time now.

Ignoring the mocks in assets/src/components seems to resolve the issue, but I'd like to get a second opinion from @johnwatkins0 on this.