Fluent: improve dropzone

[OEvgeny]

Changelog Entry

• Improved drop zone behavior and styling in Fluent theme, in PR #5328, by @OEvgeny

Description

This PR enhances the drop zone UX in the Fluent theme. The changes address issues with multiple drop zones and refine the styling to align better with Fluent design principles.

Specific Changes

1. Fixed an issue where multiple drop zones could remain visible after a drop operation
2. Updated drop zone background color to use Fluent theme variables
3. Added a transition effect for better visual feedback
4. Implemented a dragend event listener to ensure proper state reset

-

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[compulim]

Just need changelog. 😉