Floating sidebar

[lutzky]

For long pages, I'd really love a floating panel, similar to how Wikipedia does table-of-contents:

This floating TOC has some specific interesting features:

• Scrolling the main page doesn't scroll away from the TOC
• TOC can be scrolled separately if it's long enough
• Not shown if the browser window isn't wide enough
• (less important:) currently scrolled-to section of main page highlights the corresponding entry in the TOC

Obsidian has this feature too, but it's confusingly called "Outline".

However, I would want a more generic version of this; for instance, I'd like to be able to place Linked Tasks on the sidebar.

This is related to #245, but different (want contents of specific document, not across documents).

This is related to #577, I believe it's the meaty part of lhs/rhs.

[mjf]

Once speaking about generic solution, I would also like if (optional, opt-in) mode to overflow over the page content ("modal" mode) was supported for all 5 placements with good CSS (margins, padding, min/max width/height etc.) so that we could override it, if needed.