Reading layout properties changes behavior in slint code

[hunger]

Bug Description

The following slint code displays a blue rectangle. When you comment out the debug(...) in line 3, the rectangle vanishes.

I hope the example is pretty minimal :-)

Reproducible Code (if applicable)

component StringWidget inherits VerticalLayout {
    init => {
        debug("INIT", self.preferred-height);
    }

    Rectangle {
        height: 100px;
        width: 100px;
        background: blue;
    }
}

export component Test inherits Window {
    xxx := Rectangle {
        property <bool> open: true;
        
        VerticalLayout {
            Rectangle {
                clip: true;
                height: xxx.open ? self.preferred-height : 0px;

                StringWidget {
                }
            }
        }
    }
}

Environment Details

• Slint Version: master
• Platform/OS: Linux
• Programming Language: Slint
• Backend/Renderer: FemtoVG

I can also reproduce the issue in slintpad version 1.9.2.

Product Impact

No response

[ogoffart]

Smaller testcase:

component StringWidget inherits VerticalLayout {
    init => {
        self.preferred-height;   // **** Comment this line to observe a difference
    }

    Rectangle {
        height: 100px;
        width: 100px;
        background: blue;
    }
}

export component Test inherits Window {
    Rectangle {
        height: self.preferred-height ;
        clip: true;
        StringWidget { }
    }
   
}

Note that if StringWidget ends up inlining (eg, writing StringWidget { Rectangle{} }) it also changes the behavior.
I think the issue here is that the access to self.preferred-height force the property to be materialized and then it is visible when computing constraints in the default_geometry pass.