When moving by unit, UiaTextRange keeps track of the last character to be the "end of the buffer". This provides significant performance improvements in our accessibility model for word navigation in particular.

However, it is still possible to create a UiaTextRange outside of that optimized buffer size. This has lead to a few bugs like #6402 and ##5243 across Terminal and ConHost.

This issue tracks taking the time to properly test this and polish this architectural change. Especially considering that this breaks a significant number of tests that already exist.