TOC sidebar flickers on item click

Clicking a link in the left table of contents sidebar causes a brief visual flicker (looks like a re-render/re-paint of the sidebar)

https://github.com/user-attachments/assets/08b03ebd-ae06-4498-beeb-01683a68e451

Start at the table-of-contents sidebar and its item-click entry point, then reproduce the flicker by clicking a left-sidebar link while watching the rendered sidebar. Trace which state or navigation update causes the re-render or repaint. Done means clicking any TOC item changes the view without a visible sidebar flicker, with the existing widget behavior preserved.

Written by the indexing model from the issue text.