Fix more target content offset edge cases

[bryankeller]

Details

This fixes more edge cases. One thing that simplified things quite a bit is that when inserting / deleting items, we only try to preserve the content offset if we're scrolled to the very top or bottom. If we're somewhere in the middle, we just let the insert / delete affect the content offset.

Related Issue

N/A

Motivation and Context

See above

How Has This Been Tested

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.
• [ ] I have added tests to cover my changes.