Fix markdown header preview sticky button position

What does this MR do and why?

Fixes markdown header preview sticky button position in a panel.

Screenshots or screen recordings

Before	After
FF on
before	after
FF off

How to set up and validate locally

1. Go to https://gdk.test:3000/flightjs/Flight/-/issues
2. Open an work item
3. Scroll a bit down (important step!)
4. Click on the Edit button in the sticky header
5. Add content to the Markdown editor
6. View preview
7. Scroll down

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Closes #574867 (closed)