Sidebar remains static when user scrolls

[DeniseColbert]

E.g. child pages of Gen Res section but should act this way on all pages with a sidebar

[TazHinkle]

https://wp1.freeukgen.org.uk/scroll-demo
See also my comments on Slack.

I'm not moving this to review because I haven't adjusted any pages that are being kept (and don't have a full list of what pages should have this feature). My suspicion is that you would have to adjust the fixed items for every page rather than changing one template.