Site Editor: Fix classic theme site preview flickering

[karthikeya-io]

What?

Closes #69474

Fix classic theme site preview flickering

Why?

To prevent flickering and layout shift while the page is loading

How?

Hide the site-preview until other styles are applied .

Testing Instructions

1. Activate a classic theme
2. Go to Appearance > Design
3. Check the initial load of the preview, you can also check by reloading the browser.
4. Confirm there is no appearance and disappearance of the admin bar

(Test it with Fast 4g or lower speeds in the network tab of the inspector as this issue is more prominent at lower speeds)

Testing Instructions for Keyboard

Screenshots or screencast

Before

flickering-before.mov

After

flickering-after.mov

[github-actions]

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[stokesman]

This makes sense and I like that it avoids a flash of white background before the page loads. Thanks for putting it together. 🙇

I’m hoping we can go with a solution that doesn’t use the load event because it doesn’t fire until all media on the page has loaded. Even in a media-free case, it’s later than necessary for what’s being changed in the DOM. It’s potentially much later in cases with media—although probably not too commonly or gravely.

I’ve taken a look at avoiding the load event and it wasn’t looking too simple (with JS only). My current favorite idea is to have this handled from the backend #69474 (comment).