Winui3 window cannot correctly fullscreen using win32 apis

[HO-COOH]

Describe the bug

I was using Raymond Chen's method to fullscreen a winui3 window, and found out it does not work correctly.

• If you open a winui3 window, then click the taskbar once, then make the window fullscreen, the taskbar does not disappear.
• If you make a winui3 window fullscreen, then click win key to show the taskbar and the start menu, the taskbar does not disappear either.

However, using the exact same code on a normal win32 window (created by using RegisterClass and CreateWindow) works as expected.

Using Microsoft.UI.Windowing apis on a winui3 window DO work as expected. I wonder what magic does it do and what additional win32 code requires to make it work.

Why is this important?

I need my win32 apis to work.

Steps to reproduce the bug

2025-10-18.02-50-25.mp4

As in the video, I have a win32 window and a winui3 window opened. And on clicking the buttons, it will make a window fullscreen and then restore after 3 seconds.

You can see how the taskbar does not dismiss with the winui3 window.

My code:

class SupportFullScreen
{
	HWND m_hwnd;
	WINDOWPLACEMENT m_placement;
	bool m_isFullScreen = false;

	void SetFullscreen();
	void ExitFullscreen();
public:
	void SetHwnd(HWND hwnd);

	void Toggle();
};

void SupportFullScreen::SetHwnd(HWND hwnd)
{
	m_hwnd = hwnd;
}

void SupportFullScreen::SetFullscreen()
{
    DWORD dwStyle = GetWindowLong(m_hwnd, GWL_STYLE);
    MONITORINFO mi = { .cbSize = sizeof(mi) };
    if (GetWindowPlacement(m_hwnd, &m_placement) &&
        GetMonitorInfo(MonitorFromWindow(m_hwnd, MONITOR_DEFAULTTOPRIMARY), &mi)
    ) 
    {
        SetWindowLong(m_hwnd, GWL_STYLE, dwStyle & ~WS_OVERLAPPEDWINDOW);
        SetWindowPos(m_hwnd, 
            HWND_TOP,
            mi.rcMonitor.left, mi.rcMonitor.top,
            mi.rcMonitor.right - mi.rcMonitor.left,
            mi.rcMonitor.bottom - mi.rcMonitor.top,
            SWP_NOOWNERZORDER | SWP_FRAMECHANGED
        );
    }
}

void SupportFullScreen::ExitFullscreen()
{
    DWORD dwStyle = GetWindowLong(m_hwnd, GWL_STYLE);
    SetWindowLong(m_hwnd, GWL_STYLE, dwStyle | WS_OVERLAPPEDWINDOW);
    SetWindowPlacement(m_hwnd, &m_placement);
    SetWindowPos(
        m_hwnd, 
        NULL, 
        0, 0, 0, 0,
        SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_FRAMECHANGED);
}

void SupportFullScreen::Toggle()
{
    m_isFullScreen ? ExitFullscreen() : SetFullscreen();
    m_isFullScreen = !m_isFullScreen;
}

Actual behavior

No response

Expected behavior

No response

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.8.2: 1.8.251003001

Windows version

Windows 11 (24H2): Build 26100

Additional context

Full repro here

[castorix]

On my Windows 10 OS (22H2), it works in C# if I call first :

bool bRet = SetProp(m_hwnd, "NonRudeHWND", new IntPtr(0));

("NonRudeHWND" is documented in MSDN at https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/nf-shobjidl_core-itaskbarlist2-markfullscreenwindow)

[HO-COOH]

@castorix This WORKS on windows 11. Very much appreciated.

Now my question for winui team is, why would a winui3 window requires this seemingly hacking solution?