## Summary of the Pull Request
This PR clamp the "new rows" scrolling value to a positive number. We can't create a negative number of new rows. It also adds a test.
## References
## PR Checklist
* [x] Closes#5540
* [x] I work here
* [x] Tests added/passed
* [n/a] Requires documentation to be updated
## Detailed Description of the Pull Request / Additional comments
The origin of this bug is that as newlines are emitted, we'll accumulate an enormous scroll delta into a selection region, to the point of overflowing a `SHORT`. When the overflow occurs, the `Terminal` would fail to send a `NotifyScroll()` to the `TermControl` hosting it.
For this bug to repro, we need to:
- Have a sufficiently large buffer, because each newline we'll accumulate a delta of (0, ~bufferHeight), so (bufferHeight^2 + bufferHeight) > SHRT_MAX
- Have a selection
## Validation Steps Performed
* Dustin verified this actually
* Created a new insane test case
1ce86f8f1a