Update the scrollbar postiton on scrollToMark

[zadjii-msft]

When I moved this into ControlCore, I forgot that UserScrollViewport is usually triggered by the scrollbar updating, so it doesn't ask the UI to update. Since this logic is in ControlCore, it's sorta in a weird place where it needs to communicate both up and down:

• update the Terminal's viewport position
• update the TermControl's scrollbar position

Checklist:

• Closes a bug bash bug
• Missed in Experimental: add support for scrollbar marks #12948
• See also megathread: Scrollbar Marks #11000

[lhecker]

Why doesn't _terminalScrollPositionChanged simply call UserScrollViewport?

[zadjii-msft]

Why doesn't _terminalScrollPositionChanged simply call UserScrollViewport?

That's maybe an artifact of an older day. In the past there were definitely some weird feedback loops you could get into if you triggered a scrollbar update, from a scrollbar update, if that makes sense. So we separated out the scrolling actions - one for viewport updates that were initiated by the terminal itself (e.g. emitting newlines), and another for updates that were initiated by the user scrolling the viewport (e.g. with the scrollbar). These were _terminalScrollPositionChanged and UserScrollViewport, respectively.

This particular one is a weird case. This is basically a user initiated scroll, so typically those happen by updating the scrollbar itself, in TermControl, which eventually triggers a UserScrollViewport to tell the Terminal that the viewport is in a new place. However, the TermControl doesn't have the performant ability to do this lookup (hence, the move of this method from AppActionHandlers to ControlCore). So we go down into control core, but now it's sitting halfway between the scrollbar and the actual buffer, and now both need to be updated.

[ghost]

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[ghost]

🎉Windows Terminal Preview v1.15.186 has been released which incorporates this pull request.:tada:

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