feat(cover-block): add parallax effect support to video backgrounds

[Takshil-Kunadia]

What?

Closes #52585

This PR adds "Fixed background" (parallax effect) support for video backgrounds in the Cover block, similar to the existing functionality available for image backgrounds.

Why?

Currently, the Cover block allows setting an image background to be "Fixed" - so the image remains in place while the content scrolls over the top (parallax effect). However, this option was not available for video backgrounds.

Users have requested the ability to use the same parallax/fixed effect with video backgrounds to create more engaging visual experiences.

How?

1. Extended the "Fixed background" toggle to also show when a video background is selected (previously only shown for images).
2. Since videos don't support background-attachment: fixed like images do, the parallax effect for videos is achieved using:
   • position: fixed on the video element to fix it relative to the viewport
   • clip-path: inset(0) on the Cover block container to create a clipping context that constrains the fixed video within the block's bounds

Testing Instructions

Testing Instructions for Keyboard

1. Open a post or page in the editor.
2. Insert a Cover block.
3. Add a video as the background (click "Upload" or "Media Library" and select a video file).
4. In the block settings sidebar under "Settings", toggle on "Fixed background".
5. Add some content inside the Cover block.
6. Add more blocks above and below the Cover block to enable scrolling.
7. Preview the page and scroll - the video should remain fixed while the content scrolls over it.
8. Verify that the video stays within the Cover block bounds and doesn't overflow.

Screenshots or screencast

Before(No support for parallax):-

Screen.Recording.2025-12-31.at.5.15.54.PM.mov

After:-

Screen.Recording.2025-12-31.at.5.16.26.PM.mov

[github-actions]

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Warning: Type of PR label mismatch

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• Required label: Any label starting with [Type].
• Labels found: [Package] Block library.

Read more about Type labels in Gutenberg. Don't worry if you don't have the required permissions to add labels; the PR reviewer should be able to help with the task.

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Co-authored-by: Takshil-Kunadia <[email protected]>
Co-authored-by: annezazu <[email protected]>
Co-authored-by: jordesign <[email protected]>

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