What's interesting is that (at the bottom of the stack of JavaScript turtles) xterm.js depends on pty.js. Windows does not provide a pseudoterminal API like Linux, so there is no official way of writing a terminal emulator that behaves like the builtin one (conhost.exe.) Because of this, pty.js uses winpty-agent.exe and winpty-agent.exe does its magic by starting a hidden conhost.exe in the background, effectively screen-scraping it, then writing a sequence of xterm-like control sequences to a pipe that will recreate the state of conhost's screen buffer.

This works well enough in practice, but it relies on a few strange hacks. For example, it writes a marker into the console's backbuffer to detect window scrolling, and it enables mark-mode in the hidden conhost window to "freeze" it while it queries for changes. Also, it's not very robust to changes in the console implementation, so it broke for a little while due to the console changes in Windows 10 (though this was fixed quickly.)

Because of this, it's strange to see Microsoft using it in an official product. I'd love to see them add a proper pseudoterminal (pseudoconsole?) API to Windows 10. There is clearly a need for it.