Add support for the DECALN escape sequence

[j4james]

Summary of the Pull Request

This adds support for the DECALN escape sequence, which produces a kind of test pattern, originally used on VT terminals to adjust the screen alignment. It's needed to pass several of the tests in the Vttest suite.

PR Checklist

• Closes Add support for the DECALN escape sequence #3671
• CLA signed. If not, go over here and sign the CLA
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Detailed Description of the Pull Request / Additional comments

To start with, the ActionEscDispatch method in the OutputStateMachineEngine needed to be extended to check for a new intermediate type (#). Then when that intermediate is followed by an 8, it dispatches to a new ScreenAlignmentPattern method in the ITermDispatch interface.

The implementation of the ScreenAlignmentPattern itself is fairly simple. It uses the recently added PrivateFillRegion API to fill the screen with the character E using default attributes. Then in addition to that, a bunch of VT properties are reset:

• The meta/extended attributes are reset (although the active colors must be left unchanged).
• The origin mode is set to absolute positioning.
• The scrolling margins are cleared.
• The cursor position is moved to home.

Validation Steps Performed

I've added a screen buffer test that makes sure the DECALN sequence fills the screen with the correct character and attributes, and that the above mentioned properties are all updated appropriately.

I've also tested in Vttest, and confirmed that the first two pages of the Test of cursor movements are now showing the frame of E's that are expected there.

[zadjii-msft]

Yea this one's pretty straightforward. Thanks!

[ghost]

Hello @zadjii-msft!

Because this pull request has the AutoMerge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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[ghost]

🎉Windows Terminal Preview v0.8.10091.0 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes
• Store Download

[DHowett-MSFT]

🎉 Once again, thanks for the contribution!

This pull request was included in a set of conhost changes that was just
released with Windows Insider Build 19603.