Add support for LNM (Line Feed/New Line Mode)

[j4james]

Description of the new feature/enhancement

When LNM is set, the linefeed controls (LF, FF, and VT) will also trigger a carriage return. When reset they, they won't. This is essentially the inverse of our existing console mode, DISABLE_NEWLINE_AUTO_RETURN.

However, LNM has an additional effect on input. When set, the Return key generates both a carriage return and a line feed. When reset it only generates the carriage return.

It's quite widely implemented, but I don't think VTE supports it, which suggests it's probably not that widely used. However, it is a requirement for meeting VT level 1 conformance, which is why I would like for us to support it.

Proposed technical implementation details (optional)

I had originally thought we could map it directly to the DISABLE_NEWLINE_AUTO_RETURN mode, since we're already using that to determine how linefeed controls are interpreted. However, when a Windows console app has DISABLE_NEWLINE_AUTO_RETURN reset (which is the equivalent of LNM being set), we don't typically want the Return key to behave differently.

So my idea was this: We add a new input mode that specifically handles the LNM behavior for the Return key, which by default is disabled. And we treat the DISABLE_NEWLINE_AUTO_RETURN mode as an inverse alias for the LNM output behavior (as we already do).

When those two match, we're in a valid VT state, and we can use the LNM mode to toggle them both at the same time. But when they're out of sync (which is the default state for a Windows console app), we just act as if the LNM mode is not supported, i.e. we don't respond to any attempts to change it, and DECRQM reports the mode as unknown.

Does that seem like a reasonable approach to take?

[lhecker]

Isn't LNM basically a superset of the DISABLE_NEWLINE_AUTO_RETURN functionality? If so, why can't we have LNM override the DISABLE_NEWLINE_AUTO_RETURN setting entirely instead of checking if they're in sync?

[j4james]

The current default behavior for Windows console apps (i.e. when DISABLE_NEWLINE_AUTO_RETURN is reset) is to execute both a carriage and a line feed when LF is output, and to generate just CR when the Return key is pressed. This behavior does not match either of the LNM states.

When LNM is set, a Return key will generate both CR and LF, which a lot of console apps will register as a double key press. And when LNM is reset, you won't automatically get carriage returns when outputting *nix-style text content with \n line endings (I'm guessing that was the main reason the console added the auto-return functionality in the first place).