Gemini 2.5 Flash(developers.googleblog.com)

  how does this work when it gets to the bottom. Let's say I have a heading at the very bottom of the page. Will the "active" one be the bottom one or the top?

  What happens at the bottom of the page? 
  When you're near the bottom of the page and the last section (section3, say) starts to enter the top of the viewport (within 100px), it becomes the active one — exactly as you'd want.

  But here's the catch: if the last section is short, or you scroll all the way to the very bottom (past the heading), then no section's top may be ≤ 100px, especially if the last heading has already scrolled off the top. In that case, the last matched section (likely the one before the last) stays active, which feels wrong.

  We can fix this either by:
  Making the threshold smarter (e.g., using .bottom or checking if any part of the section is visible).
  Or better: use IntersectionObserver to handle this robustly.
 
  Why this is better:
  It triggers when a section is entering the viewport (around 30% from top).
  Works even when you scroll to the very bottom.
  It’s less sensitive to section height or viewport size.

  Show me exactly what was changed from this to the last which fixed the problem

  Perfect — let's do a line-by-line diff-style breakdown of what changed between the buggy version and the fixed version so we can pinpoint exactly what fixed the issue.

  Why this works:
  We evaluate all visible headings, not just ones intersecting a line.
  We pick the one that’s:
    just above the activation line, or
    just below it, if none are above
  Handles edge cases like top/bottom of scroll