Theme preview is bugged for all themes in the directory

[superbthemescom]

Hi!
The preview is aligned right, and 20% of the content is missing in the view on all themes in the Wp.org directory.

Here's how you reproduce it:

1. On a website with WordPress installed, go to the themes tab and click "Add new": /wp-admin/theme-install.php?browse=popular
2. Pick any theme and click "Preview".

This bug doesn't occur on wp.org when clicking the preview button, only on websites with WordPress installed.

Can't figure how to upload images, but the bug is easy to replicate anyways.

The bug is caused by following CSS:

#customize-container iframe, .theme-install-overlay iframe {
    width: 100vw;
}

Changing it to follow, fixes it:

#customize-container iframe, .theme-install-overlay iframe {
    width: 100%;
}

[joyously]

This is actually a problem in core, introduced by changeset [52647](​https://core.trac.wordpress.org/changeset/52647) for ticket ​https://core.trac.wordpress.org/ticket/54764.

[tellyworth]

Closing since this is best handled in the core ticket linked by @joyously.

[SergeyBiryukov]

Hi there, welcome to WordPress Meta Trac! Thanks for the ticket.

As the ticket linked above was closed on a completed milestone, I've transferred this report to a new one: ​#WP56097, let's continue there.