Window background cannot be transparent.

I thought it might be the same as the following issue, but I can't solve it.

https://bugreports.qt.io/browse/QTBUG-112473

 

I using Qt 6.6 dev on Windows 10/11.

And developing Qt Qucik application.

 

Part of code:
ApplicationWindow {
    id: root
    width: 640
    height: 480
    visible: true
    color: "transparent"

I set color to transparent. 

I expected the background to be transparent, but it was drawn white.

 

So I added the following code:

    MyUtils

{         id: myutils     }

    onVisibleChanged:

{         myutils.fillBackgroundBlack()     }

and

void MyUtils::fillBackgroundBlack()

{     QWindowList wins = QGuiApplication::topLevelWindows();     if (wins.empty()) return;     HBRUSH hbrush = CreateSolidBrush(RGB(0, 0, 0));     HWND hwnd = (HWND)wins[0]->winId();     RECT rc;     GetClientRect(hwnd, &rc);     HDC hdc = GetDC(hwnd);     FillRect(hdc, &rc, hbrush);     ReleaseDC(hwnd, hdc);     DeleteObject(hbrush); }

 

This way I was able to make the window transparent by forcing the background to be filled with black.

 

I would expect to be able to make the background transparent without any special hacks like this.

 

This problem has been occurring since the following fix.

https://codereview.qt-project.org/c/qt/qtbase/+/421952

 

expected:

actual: