[Change] Move 'On this page' on Left Sidebar

[ribbal]

When the 'On this page' list snaps to the left sidebar (when reducing the browser screen width), it shifts the main 'Page index' down. This feels a little disjointed.

The page index should remain in its fixed position and the 'On this page' should display below it, which feels a much smoother/natural.

Also, displaying it below the page index would be more consistent with any sidebar plugins (for an example, see the enhanced referencing pages plugin #527)

[redimp]

Hey @ribbal, thanks for bringing this up and for submitting a PR!

I agree that the shift when resizing is a bit disturbing. But this is a price I'm willing to pay for having the more important information on top: when the "On this page" list is displayed, it refers to the page you are currently reading, so it should be the first thing in the sidebar. The page index is more general and moves down in this case.

[ribbal]

Hi @redimp, thanks for the explanation! That makes sense, and I agree that keeping the "On this page" section at the top is a reasonable trade-off since it's the most contextually relevant navigation for the current page.

I tend to keep my pages relatively short—usually no more than a couple of screenfuls of content—so I rarely use the "On this page" section. In practice, I only really notice it replacing the "Page index" when the browser's developer tools are open and the content area becomes narrower. So it's not a major issue, and I'm happy to live with that behavior.

I'll leave the issue open for another week or so, and if there isn't any additional interest in this change, I'll close both the issue and the PR.