Fix auto-scroll should not pause on invisible activities

[compulim]

Fixes #3943.

Changelog Entry

Fixed

• Fixes #3943. Auto-scroll should skip invisible activities, such as post back or event activity, by @compulim, in PR #3945

Description

When we compute when to pause the scroll, we based on the offsetTop of "last acknowledged activity" (LAA). When the end-user send a message, the LAA will be that message.

If the LAA is an invisible activity, such as post back, message back, or event activity, we will not able to get the offsetTop because the LAA is not rendered.

This fix is to compute the offsetTop based on the "first visible activity after the LAA", instead of LAA itself.

Design

Specific Changes

• Modify the scroller callback to find out "first visible activity after the LAA" and use it for all computations
• Add 3 tests to make sure invisible activities from both side (bot/user) works with both "pause auto-scroll behavior" (num activities or num pages)

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[cwhitten]

a couple comments

[compulim]

Done. Ready for another round.