Top round transparent window - qt

Question

I'm currently learning QML and I want to create a top round transparent window.
I've build something that looks like that, but it seems wrong for multiple reason.
Here's my code:
Window {
id: app
visible: true
width: 70
height: 70
flags: Qt.Window | Qt.FramelessWindowHint | Qt.WA_TranslucentBackground
Rectangle {
anchors.fill: parent
radius: parent.width / 2.0
color: "black"
MouseArea {
property point clickPos: "1,1"
anchors.fill: parent
drag.target: parent
onPressed: {
clickPos = Qt.point(mouse.x,mouse.y)
}
onPositionChanged: {
var delta = Qt.point(mouse.x-clickPos.x, mouse.y-clickPos.y)
app.x += delta.x;
app.y += delta.y;
}
onDoubleClicked: app.close()
}
}
}
using these flags in the main :
QQuickWindow::setDefaultAlphaBuffer(true);
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
The main problem is that the background is not transparent.
I think it is because the 'round' rectangle is fully painted !?
I've tried multiple flags (Qt.Tool, Qt.Transparent, ...) but none works.
I was wondering if I started well to do what I want (I think I don't) and what is the best way to do it.
I've seen the clipping property for the qml item but I also see there's performance issues. I don't know if it's a good idea to use that property.
I'm running on Qt 5.10 and Win7 using MSVC as compiler.
Thank you
EDIT: Adding transparent background color to the window

Answer 1

Adding an answer, just so I can post an image to prove to you that all you need is to set the color:
Window {
id: app
visible: true
width: 70
height: 70
flags: Qt.Window | Qt.FramelessWindowHint
color: "#00000000"
Rectangle {
anchors.fill: parent
radius: parent.width / 2.0
color: ma.pressed ? "red" : "black"
MouseArea {
id: ma
property point clickPos: "1,1"
anchors.fill: parent
drag.target: parent
onPressed: {
clickPos = Qt.point(mouse.x,mouse.y)
}
onPositionChanged: {
var delta = Qt.point(mouse.x-clickPos.x, mouse.y-clickPos.y)
app.x += delta.x;
app.y += delta.y;
}
onDoubleClicked: app.close()
}
}
}
And the result:
I didn't use any of the flags you are setting from C++, maybe setDefaultAlphaBuffer() is breaking it for you.

Answer 2

I've figured it out.
Searching more deeply on the net and thx to #dtech, I found this article
It was the exact same problem as me. But without #dtech, I would never have thought about the graphic card problem, which led me to this solution.
It seems that you need to have the aero mode enable on windows in order to be able to use transparency on Qt.
I activated the aero mode and then retried the given solution (the one of #dtech), it works very nice.
EDIT: It's a well known "bug" on Qt
Now that I have the solution, it seems obvious but I didn't think about it before.
Thx everyone