Header submenus theme component pushes bottom of sidebar out of the screen

[awesomerobot (Kris)]

@awesomerobot

One solution that comes to mind for improving the case of the non-sticky header is to:

1. Check the height of the custom header
2. Include the custom header height in --header-offset
3. De-increment the custom header height used in the --header-offset calculation based on scroll position. This will more gradually adjust the sidebar height in a way that the .docked class can not.

This has been implemented here (big thanks to @david):

https://github.com/discourse/discourse/pull/21059

This should solve sidebar, chat and some other issues with custom header content.