ConPTY Passthrough mode

[be5invis]

Modern apps won’t read the hidden character grid and do everything in VT. So why not an API/console mode to tell Console Host to completely throw away that?

[DHowett-MSFT]

@zadjii-msft was looking into this. The main issue, if I recall correctly, is that we need to trash the entire buffer when something enters or exits “passthrough” mode. It’s also only truly applicable when there is a connected pseudoconsole session.

[DHowett-MSFT]

And the reason one console might enter and exit passthrough mode multiple times is that you may run coolNewThing.exe from CMD, and perhaps it might launch a further process that needs legacy support.

Additional concerns: if you have a tree of four processes, each of which wants passthrough to be different, should the ones that are doing ReadConsoleOutput be able to read the buffers from the other legacy/non-passthrough ones?

It’s complicated when you get into compatibility discussions. 😄

[zadjii-msft]

Yea, I tried getting this working for like a day last year, but it's something I've wanted to work on for a while.

As Dustin mentioned, there'd be real weirdness moving between passthrough mode and non-passthrough mode. However, I think it might still be something good to investigate.

[zadjii-msft]

From #1985:

I've discussed this a couple times on different threads before, but I think I never made a real issue for it.

The idea of conpty passthrough mode is that a commandline client application
that knows it's only going to use VT sequences (and not the API) to modify
the console could set a special mode, ENABLE_PASSTHROUGH_MODE. If conpty is
active, the console would then stop rendering itself over conpty, and anything
that was written to the console would go straight to the terminal.

• This would work especially well for something like wsl, where it's only ever going to be talking VT.
• It would not work for cmd.exe, because of cmd.exe's heavy reliance on the API.
• Windows Powershell again needs the API pretty heavily, but maybe Powershell
  Core, which is cross-platform, could make use of it.

This has some rough edges that need to be sorted out.

• What happens when the app exits, and passthrough mode is turned off? The
  terminal and conpty's buffers would not be equivalent anymore!
  • I've been toying with the idea of having conpty both pass the sequences
    through, and also process them itself, so that the terminal and conpty stay
    in sync.
• What happens when someone tries to call Console API's in passthrough mode?
  • I'm thinking we just cause them to fail. All save for Get/SetConsoleMode and reading input / writing output.
  • We'll also probably need to be able to read resize events.

This needs a real spec written, but it probably needs prototyping done before that.