Active Link css transition flickering

Which project does this relate to?

Router

Describe the bug

When styling the active link with a color transition, the previously active link is occasionally styled incorrectly. It briefly flickers with the previous colors for less than a second. This happens rarely (maybe 1 in 30 navigations). I’m not sure whether the bug is caused by the tanstack router or by CSS itself.

Your Example Website or App

https://github.com/idonotknowwhoiam/tanstackrouter-repro

Steps to Reproduce the Bug or Issue

1. Clone the repository.
2. Start the server (dev/preview).
3. Navigate between pages, occasionally refreshing the browser (this may or may not influence the issue idk).

Expected behavior

The active link should switch styles without any flickering.

Screenshots or Videos

https://github.com/user-attachments/assets/f503d530-4646-41ba-a5c4-18850d70a4f7

Platform

• Router / Start Version: 1.139.12
• OS: macOS
• Browser: Chrome, Arc
• Browser Version: 142.0.7444.177
• Bundler: Vite
• Bundler Version: 7.1.7

Additional context

No response

Start by cloning the linked tanstackrouter-repro application and reproduce the intermittent flicker while navigating between pages in the listed browsers. Trace the router's active-link styling behavior alongside the CSS transition; done means identifying whether the router or CSS causes the flicker and preventing it during navigation.