Keys using escape sequences (arrow keys, home/end) don't work with Bash on Windows

[rprichard]

When running Bash-on-Windows under winpty, keys encoded using escape sequences have no effect.

I tested with the latest Windows 10 preview release, 14342. It looks like there is a new, undocumented mode flag on the console input handle, 0x200. In that mode, most key-down and key-up key events are still generated as before, but for keys that use escape sequences on Ubuntu, the Windows console instead generates a series of KEY_EVENT records encoding the sequence.

The escape sequence replaces the normal key-down record, which is not generated. The key-up record is generated as before. In the records, the bKeyDown and wRepeatCount fields are TRUE and 1 (as before), and the UnicodeChar has the appropriate character, but all other fields are zeroed. (In particular, wVirtualScanCode and wVirtualKeyCode are 0.)

Example from winpty-agent --show-input --escape-input for Ctrl-Alt-Shift-UP:

key: dn rpt=1 scn=0 0x0 ch=0x1b
key: dn rpt=1 scn=0 0x0 ch='['
key: dn rpt=1 scn=0 0x0 ch='A'
key: up rpt=1 scn=72 Enhanced-LAlt-LCtrl-Shift-UP ch=0

Obviously, if Windows is going to behave this way, then winpty needs to detect the 0x200 console mode and emit escape sequences.

The interesting design question is whether winpty should send its terminal input directly to Ubuntu, or whether it should continue decoding input, but reencode escape sequences.

It looks like Windows currently ignores modifier keys when escaping. If winpty were to match Windows' encoding, it would merely have to map virtual keys to an escape string. Keys include: ESC, F1-F12, INS, DEL, HOME, END, PGUP, PGDN, UP, DN, LEFT, RIGHT. That might be all of them.

[rprichard]

winpty must decode sequences if it is to generate the key-up event records, though I suppose bash.exe probably doesn't care about those records.

[mintty]

I've just tried winpty as a gateway between mintty and Ubuntu bash on Windows, see mintty/mintty#558 (comment).
In this use case, it would not be necessary to generate console-style events for individual key sequences because the client side (Ubuntu on Windows) wants terminal-style interaction just like mintty. Is there a way to pass all escape sequences transparently so they can simply work without reiterating the huge set of terminal controls for winpty?

[rprichard]

Is there a way to pass all escape sequences transparently so they can simply work without reiterating the huge set of terminal controls for winpty?

There isn't a way to do that now, but it'd be easy to add, and it would make sense to use that option with mintty. Most likely, in that new mode, winpty would not bother generating any KeyUp events. I don't think WSL bash cares about them.

I suspect that mouse input still wouldn't work, though, unless the user manually passed --mouse to winpty to enable mouse input. winpty screen-scrapes a console buffer, so it won't see the escape sequence that an Ubuntu command-line program outputs to enable mouse input.