Remove sticky-menu class preventing full admin page scroll

[johngodley]

If you have a long admin menu it's not possible to scroll down when Gutenberg first loads.

This is caused because of #9567 adding is-fullscreen-mode to the page body class. This causes core WordPress to add sticky-menu to the page, which sets the admin menu to position: fixed.

Resizing the window causes Gutenberg to re-apply body class values, and sticky-menu is then lost.

More information can be found in this comment.

This PR modifies the FullscreenMode component to remove sticky-menu when it first starts to counteract adding is-fullscreen-mode in PHP.

Fixes #9996

How has this been tested?

1. Create plugin described here to extend your admin menu
2. Verify that on page load it's possible to scroll to the bottom of the admin menu

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[johngodley]

Noting that not adding is-fullscreen-mode in PHP also works, but then there is a noticeable UI jump when a page is first loaded.

I'm also not exactly sure if this is the best place and I appreciate the component may not be specific to WordPress.

[tofumatt]

I think this needs to add the class back when fullscreen mode is left, and this probably warrants a test too.

[noisysocks]

Hi @johnbillion! What's the status of this PR?

[tofumatt]

I think you meant @johngodley 😉

[noisysocks]

🤦‍♂️

[johngodley]

What's the status of this PR?

I need to address the point raised above - will circle back.

[youknowriad]

LGTM 👍 And thanks for the tests.

[johngodley]

Do the changes look ok to you @tofumatt ?

[tofumatt]

Works for me!