Alt arrow keys on numpad transmit extra characters in conhost

[j4james]

Windows Terminal version

Commit 63c3573

Windows build number

10.0.19045.3803

Other Software

No response

Steps to reproduce

1. Open a WSL bash shell in conhost or OpenConsole.
2. Make sure NumLock is off.
3. Run showkey -a
4. Press Alt+RightArrow on the numeric keypad.

Expected Behavior

You should just see the VT escape squence for Alt+RightArrow:

^[[1;3C          27 0033 0x1b
         91 0133 0x5b
         49 0061 0x31
         59 0073 0x3b
         51 0063 0x33
         67 0103 0x43

Actual Behavior

The escape sequence is followed by an extra ♠ character, so it looks like this:

^[[1;3C          27 0033 0x1b
         91 0133 0x5b
         49 0061 0x31
         59 0073 0x3b
         51 0063 0x33
         67 0103 0x43
♠       226 0342 0xe2
        153 0231 0x99
        160 0240 0xa0

The problem is that the combination of the Alt key with a digit on the numeric keyboard causes it to be interpreted as an Alt-Numpad input, in addition to the VT key sequence. So when the Alt key is released, it transmits character 6 (the RightArrow digit) in codepage 437, namely ♠.

This is similar to issue #3101, only that was in Windows Terminal, and I think the underlying cause is possibly different.