Overflow not hidden - allows right scrolling on header in Firefox

[cbgraphics]

When you scroll right while holding the cursor over the custom header it allows you to scroll right, past the content.

Suggested Fix
This can be fixed by adding:

overflow-x:hidden;

to the html element or primary container.

[cbgraphics]

FYI. I applied left:0; to the site header to force it into the correct position.

[lcrdd]

Added left: 0 as per cbgraphics suggestion

[lcrdd]

Just realized this is the same issue as in 38543. I noticed that removing display: flex; also fixed it and wasn't sure why flexbox was in use here, but looks like from the other ticket that it was a leftover. Anyway, I just went to a class about contributing and was testing the waters here! May I mark as duplicate?

[davidakennedy]

Thanks for the report @cbgraphics!

Thanks @lcrdd! Welcome! You can mark tickets as duplicates – I've done that here. :)

This issue is being worked on in #38543. Feel free to take a look at other bundled theme tickets if you want to dive in: https://core.trac.wordpress.org/component/Bundled+Theme

You can always ask questions on Slack in #core-themes too.