ConPTY: Avoid WINDOW_BUFFER_SIZE_EVENT when the viewport moves

[lhecker]

SetConsoleWindowInfoImpl calls PostUpdateWindowSize, which emits a
CM_SET_WINDOW_SIZE event, which causes _InternalSetWindowSize to be
called, which calls ScreenBufferSizeChange which then finally emits a
WINDOW_BUFFER_SIZE_EVENT event into the client input buffer.

This messes up applications like which make use of
WINDOW_BUFFER_SIZE_EVENT to perform potentially lossy operations.
In case of SSH this results in a resize (SIGWINCH) of the server-side
screen which similarly may result in a response by the shell, etc.
Since that happens over networks and is async, and because our conhost
VT viewport implementation appears to have a number of subtle bugs,
this results in duplicate output lines (sometimes hundreds).

Under Windows Terminal this issue is not as apparent, since ConPTY has
no viewport that can be moved and no scrollback. It only appears as an
issue if a terminal application reacts poorly to the SIGWINCH event.

Closes #15769

Validation Steps Performed

• Set a breakpoint in SynthesizeWindowBufferSizeEvent
• Launch WSL and cause the viewport to move down
  No calls to SynthesizeWindowBufferSizeEvent ✅
• Execute tput reset
  Input line moves to row 0 ✅

[zadjii-msft]

Wait does this do anything about #281?

[carlos-zamora]

Thanks for mentioning and briefly going over this in Triage 😊

[lhecker]

Wait does this do anything about #281?

No, that'd be a breaking change at this point, so this PR just avoids calling SetConsoleWindowInfo.
...but I think we should do that breaking change anyways very soon. It's only been 6 years since WINDOW_BUFFER_SIZE_EVENT was broken, but it's been >20 years before that. We should revert it, introduce separate messages and opt-ins or similar. The longer we wait the worse it gets IMO. (Especially since that the issue was reported 5 years ago, which is a bit of an oof for us.)