Improvement: Sidebar briefly flashes fully expanded on page navigation

[alankritdabral]

Page URL

https://precice.org/tutorials.html & https://precice.org/docs.html

What is wrong?

What is wrong?
When navigating between pages using the left sidebar the page performs a full reload and the sidebar briefly renders in its raw expanded HTML state before the navgoco is initialized. This creates a noticeable flicker where multiple sidebar sections appear open for a split second. And if your internet is slow its more noticeable.

Steps to reproduce:

• Open a page with the left sidebar, for example https://precice.org/tutorials.html or https://precice.org/docs.html
• Click a different link inside the sidebar
• Watch the sidebar during page load

Expected behavior:
Only the active branch should appear open, without any visible flash of the fully expanded navigation tree.

Actual behavior:
During the page transition, nested sidebar lists briefly appear expanded before JavaScript collapses them again.

Notes:
The issue appears to be caused by a flash of uninitialized content. The sidebar HTML is rendered first in _includes/sidebar.html, and the collapse behavior is only applied later when navgoco is initialized in _layouts/default.html.

Relevant files:

_includes/sidebar.html
_layouts/default.html
js/jquery.navgoco.min.js
css/customstyles.css

Recommended fix:
Hide nested sidebar lists by default only when JavaScript is available, then let navgoco show the correct open branch during initialization.