Fix target content offset edge case for small content

[bryankeller]

Details

This fixes an edge case with the target content offset anchor calculation. If the content is shorter than the visible bounds, that means we're simultaneously at our topmost offset and bottommost offset. In this scenario, the previous logic would always calculate a target content offset anchor of .top. The correct logic in this scenario should be:

• Anchor is .top if our verticalLayoutDirection is .topToBottom
• Anchor is .bottom if our verticalLayoutDirection is .bottomToTop

Related Issue

Noticed a scenario where new messages don't push the content down when a thread is just getting started (short content height)

Motivation and Context

Squashin bugs

How Has This Been Tested

Example app

Types of changes

• [ ] Docs change / refactoring / dependency upgrade
• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.