terminal/parser: determine what to do when split escape sequence writes show up

Environment

Windows build number: Version 10.0.18363.535
(reproducible with OpenConsole from master)

Windows Terminal version (if applicable): n/a

Any other software?
- Simple ConPTY test code

Steps to reproduce

• Create pseudo console as seen in examples
• Copy I/O to and from serial / COM port; alternatively manually write CSI or escape sequence in multiple write operations.

Example for arrow-up key (as seen during development with serial ports):
Write 1: 0x1b
Write 2: 0x5b 0x41

Expected behavior

• Working escape sequences over multiple write operations to PTY input pipe

Example for arrow-up key:

$ showkey -a

Press any keys - Ctrl-D will terminate this program

^[[A 	 27 0033 0x1b
 	 91 0133 0x5b
 	 65 0101 0x41

Actual behavior

Due to FlushAtEndOfString being true, the escape sequence is split up and rendered wrong. E.g. arrow-up becomes:

^[       27 0033 0x1b
[A       91 0133 0x5b
         65 0101 0x41

which only renders "[A" to the PTY without the desired effect.

This is 100% reproducible, i.e. the arrow keys never work because they are always received as two reads from the (virtual) serial port.

Note: A custom build of conhost.exe (OpenConsole.exe) with FlushAtEndOfString changed to return false does correctly process split escape sequence writes.

Comments

The comment in the caller in stateMachine.cpp says the following:

        // <kbd>alt+[</kbd>, <kbd>A</kbd> would be processed like `\x1b[A`,
        // which is _wrong_).
        //
        // Fortunately, for VT input, each keystroke comes in as an individual
        // write operation.

I think both statements here are wrong, or at least not universally true. Typing Alt+[ and A on a VTE based Linux terminal emulator (e.g. gnome-terminal or xfce4-terminal) will result in the same reaction as pressing the arrow-up key.

Is there any standard that guarantees VT input sequences to happen in single writes? I tested Linux support of split escape sequence writes - it worked fine on Linux 5.4 framebuffer getty TTY and on all tested PTY terminal emulators (xterm, VTE, etc.).

Related: https://github.com/microsoft/terminal/pull/2823

Start with FlushAtEndOfString in src/terminal/parser/InputStateMachineEngine.cpp and the caller comment in src/terminal/parser/stateMachine.cpp. Reproduce the two-write escape sequence through the ConPTY path, then review related pull request #2823 and the parser behavior around split input. Done means split CSI sequences work without incorrectly combining separate Alt+[ and A input.