Clamp the new rows scrolling value to a positive number

[zadjii-msft]

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

• Closes Scroll Sometimes Stops Before Reaching Bottom of Terminal #5540
• I work here
• 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

[DHowett-MSFT]

Guess what! You got it.

Because we're hugely negative, we fail to notifyscroll.

[DHowett-MSFT]

Fixes #5540

[zadjii-msft]

I'm working on a test for this, so hopefully we can catch this if it regresses. Since this is a Terminal-Renderer-DxEngine interaction that causes this, it's a little tricky to get the test into the right state. Hopefully I can get it figured out, otherwise we might just ship this without the test.

[DHowett-MSFT]

Wow, and booting up a renderer isn't weird or bad or scary here?

[zadjii-msft]

@DHowett-MSFT nah, we don't have a thread so it's fine, and the RoundtripTests already do something similar

[ghost]

Hello @zadjii-msft!

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[ghost]

🎉Windows Terminal Release Candidate v0.11.1251.0 (1.0rc1) has been released which incorporates this pull request.:tada:

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