Add a means by which UIA clients can determine whether a build of Windows Terminal does or doesn't support notifications

[codeofdusk]

Description of the new feature/enhancement

#12358 adds support for UIA notifications, allowing screen readers and other UIA clients to easily determine exactly what new text to report without doing any of the heavy lifting themselves (diffing, etc..). However, as not all terminals support notifications, UIA clients need a way to determine when this support is available and when they should fall back to performing diffing themseles. (if clients diff even when notifications are available, we risk doing extra work and/or double-reporting incoming text).

Proposed technical implementation details (optional)

• In Use UIA notifications for text output #12358 and over email at various times, we discussed using a different class name (TermCtrl2) for situations when notifications are available. NVDA supports this new ID today, but there were concerns from JAWS.
• We also discussed sending an "API version" of sorts through a custom property. This is necessary since wt and conhost versions aren't necessarily tied to particular Windows releases/aren't reliable for feature detection.

[codeofdusk]

Blocking nvaccess/nvda#13781.

CC @carlos-zamora, @LeonarddeR.

[LeonarddeR]

It could also be considered to add an automationID (it is currently empty)

[codeofdusk]

It could also be considered to add an automationID (it is currently empty)

@LeonarddeR That idea was rejected during initial implementation.

[carlos-zamora]

Chatted with @codeofdusk. This is a feature/task. We'll chat more about it at sync but the short version is this:

• In a world where Terminal doesn't send notifications for text output...
  • NVDA reads the text buffer, performs a diff on what was stored, and reads out what changed.
  • Diffing is (unsurprisingly) pretty expensive! This is a common practice among screen readers.
• Today, Terminal sends notifications for text output to tell the screen reader exactly what is relevant...
  • NVDA would normally read these notifications, resulting in a "local echo" where it would read the notification and the result of the diff. Not good! So NVDA straight up ignores those notifications.
  • That said, these notifications uncovered a massive opportunity for screen readers! What if they just trusted the notification instead of diffing themselves? That would be a huge performance enhancement!
• The Problem: NVDA needs some kind of check to know if Terminal will send out these notifications. If that check succeeds, then they know they can trust our notifications, and they can avoid diffing altogether.
  • NOTE: NVDA already does these "checks" on UIA. There's 3 "API Levels":
    • 0 --> "GetVisibleRange" returns one range per line of text --> initial UIA implementation (pre-UIA provider refactor)
    • 1 --> "GetVisibleRange" returns one range total --> after my UIA provider refactor
    • 2 --> text formatting (i.e. color/font) is exposed --> most recent UIA implementation