Fix Admin's sidebar height to be dynamic based on visible header

[gary]

The current sticky nav bar on admin anchors past the page. If you have long list of apps you have to scroll the whole page and then the nav to get to the bottom.

Images, context, and patch available at ​https://github.com/django/django/pull/14281

[Jacob Walls]

Thanks -- accepting based on Tom's review of the ​PR.

[Carlton Gibson]

Current approach on PR is overly complex. A simpler CSS-only approach should be feasible (?)

[Hrushikesh Vaidya]

A CSS only approach ​PR

[Carlton Gibson]

I'm going to resolve this as wontfix.

The scrolling issue doesn't seem that bad: the left sidebar pauses independent scrolling at a point, scrolling in sync with the main section until reaching the bottom (of either) after that, so it's not 100% independent scrolling, but its OK. Perhaps it's Just what browsers do, and given #32204 we have the filter in-place now too, it's a minor glitch at worst.

In contrast the fixes are either too heavy (​PR14281) or else have other display issues (​PR15234) and so it's not clear there's a simple fix available (one that's really justified by the issue). Pending such actually showing up, I don't think further effort here is the best use of time.

Happy to reopen to consider patches.