Modal backdrop not covering some UI parts

[MrGeneration]

Infos:

• Used Zammad version: develop / 5.1 (as of 22nd February)
• Installation method (source, package, ..): any
• Operating system: any
• Database + version: any
• Elasticsearch version: any
• Browser + version: verified on Opera v83, cross checked on Firefox

Expected behavior:

Backdrops of modals always cover all relevant UI parts.

Actual behavior:

Some elements may no longer be covered entirely.
This affects all table header dividers to "glow"

The chat channel sample chat widget no longer is greyed out when adding or editing a chat topic

All modals that can be opened within ticket zooms cause ticket articles, text editor and the sidebar to be lighted up like a christmas tree.

Steps to reproduce the behavior:

• have current develop
• open dialogues all over Zammad and have a look

Yes I'm sure this is a bug and no feature request or a general question.