Navigation menu jumps to the bottom when opening a new page

[NathanD-GDS]

What should change

In the GOV.UK Pay documentation, navigating to a page in the left-hand navigation pane causes that pane to scroll to the bottom.

The navigation bar should not scroll like this when move when navigating between pages. It should just show the expanded version of the page you've navigated to, as in the final few frames of the GIF.

User need

This current behaviour is disorienting when you're navigating through the docs.