Library section under Media, search bar shifts position after searching in the WordPress admin area.

[abduremon]

## Issue
After installing WordPress, the search bar in the Library under Media tab section shifts position after performing a search in the admin area.

## Reproduction Steps

• Log into the WordPress admin dashboard.
• Go to Media → Library.
• Search for any media item using the search bar.
• Observed Result.

## Observed Result
After searching, the search bar jumps/shifts unexpectedly in the Library section UI.

## Expected Result
The search bar should remain stable and properly aligned after performing a search.

[abduremon]

Library section search bar shifts position after searching in WordPress admin area.

[abduremon]

The search bar position also shifts when using Grid View after selecting a media item and applying a date filter.

[abduremon]

The search bar position also shifts when using Grid View after selecting a media item and applying a date filter.

Already uploaded another attachment.

## Summary

Fixes an issue where the search bar in the Media Library (Admin grid view) shifts/jumps horizontally (moves to the left and then snaps back) when typing/removing text in the search input or applying a date filter.

### Root Cause
During search/filtering updates, the Backbone.js view shows/hides the loading spinner and an overlay element (.media-bg-overlay) to block interaction and display the loading state.

However, .media-bg-overlay was only styled within the @media screen and (max-width: 782px) media query in media-views.css. On screens wider than 782px (desktop), the element had no style definitions. Consequently, when jQuery called .show(), the overlay was displayed as a default block element (display: block) in the normal document flow. Because it sits inside the .media-toolbar container, its insertion disrupted the floated and flex layout of .media-toolbar-primary (containing the search input) and .media-toolbar-secondary, causing the layout elements to shift.

### Solution

• Defined .media-bg-overlay globally (outside of media queries) with absolute positioning. This takes the overlay out of the document flow on all screen sizes, preventing any layout reflow/shift.

## Changes

### src/wp-includes/css/media-views.css

• Moved .media-bg-overlay layout definition to a global rule so it is always absolutely positioned and overlaying the toolbar correctly on both desktop and mobile views.

## Testing

Test Case	Expected / Result	Status
Typing search queries in Media Library Grid View	Search bar remains stable, no horizontal shift	✅ Passed
Selecting media item and applying a Date filter	Grid layout remains stable, no horizontal shift	✅ Passed

Trac ticket: https://core.trac.wordpress.org/ticket/65296

## Use of AI Tools

AI assistance: No

[wildworks]

I believe this issue is a regression caused by the following style added by r61645.

.upload-php .wp-filter {
        display: flex;
}

When performing a filter search, display: block is applied to the overlay, which seems to cause unintended issues with flex layouts.

<div class="media-toolbar wp-filter"> <!-- `display: flex` is applied -->
    <div class="media-toolbar-secondary">
    </div>
    <div class="media-toolbar-primary search-form">
    </div>
    <div class="media-bg-overlay" style="display: none;"></div> <!-- `display:block` is dynamically applied here during filter search -->
</div>