Local Navigation Bar: Update scroll offset to prevent local nav bar overlapping content

[ryelle]

Fixes #586. The local nav bar overlaps content when using anchor links. There is some code in the parent theme that used to work for the sticky header, but since we unstuck the header in favor of the local nav bar, this doesn't apply anymore.

I've taken the same approach here, but put it in the Local Navigation Bar's CSS, so that it will load whenever that block is used. This adds scroll-padding-top to html, to account for the local nav bar + admin bar (if it exists).

Some child themes have added scroll-margin-top to individual elements, this will add extra space until it's removed, but I don't think that needs to block rolling this fix out.

Screenshots

Developer

With scroll-margin-top on TOC headings	Without scroll-margin-top

Forums, with the local scroll-margin-top disabled:

Plugin FAQs

[adamwoodnz]

LGTM

Visually I think it's nice to have a little more space above the linked element, but maybe it's up to individual themes to set that. For example the Dev Resources title looks better with 20px space to the local nav so that the content is vertically aligned. Modifying the local scroll-margin-top achieves this.

[ryelle]

Yeah, I think that should be up to the individual themes. I've updated the ToC heading offset to 20px, since that's part of the mu-block, and left the regular offset to exactly the heading height. I'm going to merge this so that the plugins FAQs are offset correctly, and then the margin can be removed from wporg-support-2024.