[bug]: Close megadropdown when focus leaves the header

[gfellerph]

Describe the bug
While the current focus is in the header and a megadropdown is open, the focus should always be visible. As soon as the focus leaves the header and goes to the first focusable element on the page, the focus could be hidden by the open megadropdown. As a safety measure, the megadropdown should close when focus leaves the header.

Question: should this behaviour also be the same if the focus is lost due to a click event? This will close the megadropdown on outside click which should probably not be the case.

To Reproduce
Steps to reproduce the behavior:

1. Use the keyboard to open the megadropdown
2. Press tab until the focus leaves the header

Expected behavior
The focused element should always be visible (https://www.w3.org/WAI/WCAG21/Understanding/focus-visible.html)