Docs: Clarify popstate handling to prevent UI freeze on navigation spam

[Ala-Ben-Aissia]

Summary

This PR updates the exercise instructions / solution to address a performance bug that could freeze the UI when repeatedly pressing the back/forward navigation buttons.

The Problem

Previously, every popstate update including those already served from cache was wrapped in startTransition. When the back/forward buttons were spammed, React’s concurrent scheduler queued up a large number of unnecessary transitions, causing heavy reconciliation work and freezing the browser tab.

The Fix

The instructional comments in now clarify the correct handling for each case:

• Cache hits: Update state directly (no transition).
  
• Cache misses: Fetch new content and wrap the state update in startTransition.

This distinction ensures the UI stays responsive, even under rapid navigation.

[kentcdodds]

Interesting. Do we have this problem before this exercise as well? I think we should probably update the instructions in the problem version of this exercise step to make sure that people avoid this issue. Would you like to make an update to the pull request to call out for that?

[kentcdodds]

Thank you for your interest in contributing back!

[Ala-Ben-Aissia]

Thanks for the great feedback, Kent! You're right about the UI freeze - it only happens in the caching exercise because that's where we actually reuse cached content.
I've tweaked the instructions + solution to make the distinction between cache hits and misses clearer, which should help avoid the issue. Let me know how it looks!

[kentcdodds]

Thank you for this!

[Ala-Ben-Aissia]

You're welcome