Win32 input make/break events are generated at the same time

[o-sdn-o]

Environment

Windows build number: 10.0.19041.572
Windows Terminal version (if applicable): master

In the classic windows console, you can track alphanumeric key combinations (for example, tracking the keys w, a, s, d simultaneously). This tracking cannot be done in Windows Terminal.

Steps to reproduce

Run the following code
Press a couple of alphanumeric keys at the same time, e.g. q+w+e

#include <iostream>
#include <unordered_set>
#include <Windows.h>

int main()
{
    DWORD                    Count;
    INPUT_RECORD             Reply;
    std::unordered_set<WORD> State;

    auto Input = GetStdHandle(STD_INPUT_HANDLE);

    while (WAIT_OBJECT_0 == WaitForSingleObject(Input, INFINITE))
    {
        ReadConsoleInput(Input, &Reply, 1, &Count);
        switch (Reply.EventType)
        {
            case KEY_EVENT:
            {
                //std::cout
                //    << " Down: "     << Reply.Event.KeyEvent.bKeyDown
                //    << " Repeat: "   << Reply.Event.KeyEvent.wRepeatCount
                //    << " KeyCode: "  << Reply.Event.KeyEvent.wVirtualKeyCode
                //    << " ScanCode: " << Reply.Event.KeyEvent.wVirtualScanCode
                //    << " Char: "     << Reply.Event.KeyEvent.uChar.UnicodeChar
                //    << " KeyState: " << Reply.Event.KeyEvent.dwControlKeyState
                //    << std::endl << std::flush;

                auto ScanCode = Reply.Event.KeyEvent.wVirtualScanCode;
                auto KeyDown = Reply.Event.KeyEvent.bKeyDown;
                auto Changed = KeyDown ? State.emplace(ScanCode).second
                                       : State.erase(ScanCode);
                if (Changed)
                {
                    std::cout << "chord:";
                    for (auto Key : State) std::cout << " " << Key;
                    std::cout << std::endl << std::flush;
                }
                break;
            }
            default:
                break;
        }
    }
}

Expected behavior

Key combinations are tracked (classic console behavior)

D:\sources\temp\ConsoleApplication1\Debug>ConsoleApplication1.exe
chord: 16
chord: 16 17
chord: 16 17 18
chord: 16 17
chord: 16
chord:
chord: 16
chord: 16 17
chord: 16 17 18
chord: 17 18
chord: 18
chord:

Actual behavior

Single keystrokes are tracked only (Windows Terminal behavior)

D:\sources\temp\ConsoleApplication1\Debug>ConsoleApplication1.exe
chord: 16
chord:
chord: 17
chord:
chord: 18
chord:
chord: 18
chord:
chord: 18
chord:
chord: 18
chord:
chord: 18
chord:
...

[zadjii-msft]

Yep, this is unfortunately true. Since the Terminal is using the UWP XAML Input stack, we only get three types of input events: key downs, key ups, and character received events. Since we don't get full unicode character input from just the key down/ups, we have to rely on the character events for that. But there's no character up/down events, only received. So we've got to send a fake key down/up pair immediately. I'm sure if there was a way to avoid doing this, @lhecker would have thought of it by now 😜

For the sake of linking: #4999 dealt with a lot of similar issues. At least conpty is now capable of theoretically handling this, if only the Terminal was.

[lhecker]

We could integrate the UWP based WT more deeply with our Win32 entrypoint.
That would allow us to use something like our old approach documented here.

[DHowett]

I've renamed this and iceboxed it. There's some ways in which we'll want to be compatible with the console, and a great many in which we will not.

This one requires enough work (and subversion of the "modern" input stack) that I would bin it into the "not" category, but... that's me.

[j4james]

I've just run into this issue while trying to get certain VT input modes to work in Windows Terminal, and I'm trying to work out what the actual problem is here. I get that the key down event doesn't include character data, but we've already got code in place that can generate that character data automatically (see the _CharacterFromKeyEvent method). Are there known cases where that doesn't work correctly?

But lets assume the _CharacterFromKeyEvent method isn't good enough. Couldn't we at least use the CharacterReceived handler to generate the key down events (as we're doing now), but then still use the KeyUp handler for the corresponding key up events? There may then be edge cases where the character data could be incorrect for the key up, but would that actually affect anyone? Because a typical WM_KEYUP message wouldn't include that info anyway.

And if that really is still a concern, another approach we could take would be to save the most recent character in the CharacterReceived handler (just store it in an array mapped to the scan code), and then the KeyUp handler could easily look up the corresponding character data for a particular scan code when the key is released. Even with multiple keys pressed simultaneously I would think the scan code should suffice as a unique id.

Are there more factors at play here that I'm overlooking? Or does anyone have suggestions for sample keystrokes I should be testing that would be expected to fail? Because everything I've tried so far seemed to work OK (or at least no worse than the current implementation).

[DHowett]

Are there more factors at play here that I'm overlooking?

Probably there aren't -- I think you've by this point investigated it deeper than the rest of us in recent history. We just got another dupe for it, and I am wondering if there's something we can do about it...

While we're breaking input, and all. 😁

[j4james]

I've got an old branch with my initial simple proposal (relying on _CharacterFromKeyEvent to generate the character data), which I can turn into a PR tomorrow if you'd like. It's only a couple of lines, so it's not a big deal if we need to drop it in favor of something more complicated.