Fixed sorting of part grouping and livestreaming sorting if activities are too far apart

[compulim]

Related to #5622.

Changelog Entry

Fixed

• Fixed activity sorting introduced in PR #5622, part grouping, and livestreaming, by @compulim in PR #5632

Description

Grouped sorting is broken since PR #5622 or even earlier.

If the activity is the last one to be inserted in the group (indexToInsert = -1), they will fallback to the next group of sorting.

For example:

1. Activity 1 (t=0) belongs to part grouping
2. Activity 2 (t=2) does not belongs to part grouping
3. Activity 3 (t=1) belongs to part grouping of activity 1
   • However, due to a bug, activity 3 will be appended after activity 2 (wrong), instead of after activity 1 (correct)

Specific Changes

• Updated sorting algorithm to return "index + 1" of last grouped activities (part grouping and livestreaming), instead of -1, so it will not fall to another sorting group

• 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)