Mastodon: Arrow key to cycle lightbox images also scrolls columns

[otterlove]

Steps to reproduce:

1. Add several columns and/or make the browser width smaller so a horizontal scroll bar appears.
2. Click on one of several images in a post to expand it.
3. Use the left and right arrow keys to cycle the images.

Result: the columns behind the images also shift while the key is pressed. Seen in Chrome.
Expected: Elements behind an overlay should be disabled while the overlay is active.

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• [X] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
• [ ] This bug happens on a tagged release and not on master (If you're a user, don't worry about this).

[averageunicorn]

This happens in Firefox as well.

[iliana]

I'd suggest that this is an accessibility issue, as it causes the "background" to shift under the content you're viewing unexpectedly.

[ykzts]

Resolved with #6241