ENHANCED_KEY flag missing when releasing RightAlt

[o-sdn-o]

Windows Terminal version

current main

Windows build number

10.0.19045.4894

Other Software

No response

Steps to reproduce

1. Run the following c++ code:

#include <iostream>
#include <windows.h>
int main()
{
    INPUT_RECORD rec;
    DWORD count;
    while (true)
    {
        ::ReadConsoleInputW(::GetStdHandle(STD_INPUT_HANDLE), &rec, 1, &count);
        if (rec.EventType == KEY_EVENT)
        {
            std::cout << "type: KEY_EVENT" << std::hex
                      << ", down: " << rec.Event.KeyEvent.bKeyDown
                      << ", ENHANCED_KEY: " << !!(rec.Event.KeyEvent.dwControlKeyState & ENHANCED_KEY)
                      << ", ctrl: 0x" << rec.Event.KeyEvent.dwControlKeyState
                      << ", vcod: 0x" << rec.Event.KeyEvent.wVirtualKeyCode
                      << ", scod: 0x" << rec.Event.KeyEvent.wVirtualScanCode
                      << ", wchr: 0x" << rec.Event.KeyEvent.uChar.UnicodeChar << '\n';
        }
    }
}

2. Press LeftCtrl key, press RightCtrl key, check ENHANCED_KEY flag.
3. Press LeftAlt key, press RightAlt key, check ENHANCED_KEY flag.

Expected Behavior

The ENHANCED_KEY flag is present when RightAlt is pressed and released.

The current conhost.exe is not affected by this issue.

Actual Behavior

The ENHANCED_KEY flag is absent when RightAlt is released, but is present when RightAlt is pressed.

[o-sdn-o]

It might be located here, when manually sending an Alt key release:

terminal/src/cascadia/WindowsTerminal/WindowThread.cpp

Lines 223 to 234 in 8d3f12b

	// GH#6421 - System XAML will never send an Alt KeyUp event. So, similar
	// to how we'll steal the F7 KeyDown above, we'll steal the Alt KeyUp
	// here, and plumb it through.
	if (_messageIsAltKeyup(message))
	{
	// Let's pass <Alt> to the application
	if (_host->OnDirectKeyEvent(VK_MENU, LOBYTE(HIWORD(message.lParam)), false))
	{
	// The application consumed the Alt. Don't let Xaml get it.
	continue;
	}
	}

The subsequent call to _GetPressedModifierKeys() does not return any pressed Alt keys, so the extended key flag is lost:

terminal/src/cascadia/TerminalControl/TermControl.cpp

Lines 1466 to 1475 in 8d3f12b

	// Method Description:
	// - Manually handles key events for certain keys that can't be passed to us
	// normally. Namely, the keys we're concerned with are F7 down and Alt up.
	// Return value:
	// - Whether the key was handled.
	bool TermControl::OnDirectKeyEvent(const uint32_t vkey, const uint8_t scanCode, const bool down)
	{
	const auto modifiers{ _GetPressedModifierKeys() };
	return _KeyHandler(gsl::narrow_cast<WORD>(vkey), gsl::narrow_cast<WORD>(scanCode), modifiers, down);
	}

The solution is to take the ENHANCED_KEY flag from lParam (24th bit) of the WM_KEYUP message.