Windows tty: duplicate key with alt modifier

[parkovski]

• Version: Latest neovim / node
• Platform: Win10, 18298

When pressing a key combo with alt, the non-alt key is duplicated. Because of some quirks in the way event messages are generated, this only appears when using libuv under a ConPTY. Probably the easiest way to see this is to start a Windows exe in tmux in WSL. I see the problem both in neovim and nodejs.

Here is a nodejs program that demonstrates the issue:

process.stdin.setEncoding('utf8');
process.stdin.setRawMode(true);

process.stdin.on('data', x => {
  // Ctrl-C to quit
  if (x[0] == '\u0003') { process.exit(0); }
  console.log(x.length, x);
});

process.stdin.resume();

In a regular Windows console, or in a Linux terminal with or without a PTY, pressing for example, Alt+a will show 2 '\u001ba'. In a ConPTY however, it shows 3 '\u001baa'. This affects neovim, where a <M-*> keybinding also duplicates the key, breaking these bindings.

More info is here, including a screenshot of the difference in generated console events and the place in libuv where I think the problem is.

This C++ program will print all the generated console events, if it helps. Compile with C++17 (cl /EHsc /std:c++17 console.cpp).

[bzoz]

Thanks for the report. The line you suspected was to blame, fix for the issue is in #2114