Sidebar State Affects Auto-Hiding Behavior of Menu Bar

[WantenMN]

Description

In the documentation generated by mdBook, the auto-hiding behavior of the top Menu Bar is inconsistent depending on the Sidebar's state during page refresh. This issue is noticeable on long pages, such as the Backends page.

Steps to Reproduce

1. Scenario 1: Refresh Page with Sidebar Closed

   • Open a sufficiently long page, e.g., https://rust-lang.github.io/mdBook/for_developers/backends.html
   • Ensure the Sidebar is closed (click the button to collapse the Sidebar).
   • Refresh the page.
   • Scroll down the page; the top Menu Bar auto-hides.
   • Open the Sidebar.
   • Continue scrolling down; the Menu Bar still hides with scrolling.

2. Scenario 2: Refresh Page with Sidebar Open

   • Open the same page (https://rust-lang.github.io/mdBook/for_developers/backends.html).
   • Ensure the Sidebar is open.
   • Refresh the page.
   • Scroll down the page; the Menu Bar does not hide with scrolling.
   • Close the Sidebar.
   • Continue scrolling down; the Menu Bar fails to hide.