Fix: Prevent block UI from showing above settings panel on mobile

[sarthaknagoshe2002]

Fixes: #63845

What?

This PR addresses an issue where interacting with the block settings panel on mobile reveals the underlying UI of blocks. Specifically:

1. Adjusting the padding displays the blue margin indicators.
2. Adjusting the height of the Cover block (and similar blocks like Spacer) reveals underlying UI elements.

Why?

When interacting with the settings panel, users should only see and interact with the settings themselves. Showing the block UI underneath is confusing and can lead to unintended actions or distractions. This behavior affects usability and creates a suboptimal user experience on mobile devices.

How?

Added a media query for mobile devices to the .components-popover.block-editor-block-popover class, setting z-index: -1 when the viewport width is below $break-medium. This ensures the popover doesn't overlay or interact with elements improperly on smaller screens.

Testing Instructions

1. Create a new post on mobile
2. Adjust the padding and set the height of the cover block
3. The underlying UI of the block doesn't show up above the settings panel

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: sarthaknagoshe2002 <[email protected]>
Co-authored-by: annezazu <[email protected]>
Co-authored-by: mrfoxtalbot <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.