[REG 6.2.1 -> 6.4β2] QML onLineLaidOut crashes

We recently upgraded from Qt 6.2.1 to Qt 6.4 Beta 2, and something stopped working.

onLineLaidOut() sometimes crashes now.

Here's the qml:
main.qml:

Item {
    id: mainItem
}

TestWindow.qml:

ApplicationWindow {    
    id: appWindow
    visible: true    
    Text {
        id: test
        text: "crashing label"
        onLineLaidOut: ( line ) => {
            console.log( "visible: " + visible )
            console.log( "number: " + line.number )
            console.log( "x: " + line.x )
            console.log( "y: " + line.y )
            console.log( "w: " + line.width )
            console.log( "h: " + line.height )
            console.log( "isLast: " + line.isLast )
            console.log( "implicitWidth: " + line.implicitWidth )
        }
    }
}

Here's how I instanciate it

_engine = new QQmlApplicationEngine();

_engine->load( "qrc:/Quick/main.qml" );
 
QQmlComponent component( _engine, "qrc:/Quick/TestWindow.qml" ); 
component.create( nullptr );

Here's the output:

The first time:

qml: visible: true
qml: number: 0
qml: x: 0
qml: y: 0
qml: w: 8388607
qml: h: 16
qml: isLast: true
qml: implicitWidth: 72.90625

and the second time:

qml: visible: true
qml: number: -1476398904

and then it crashes.

Depending on what I put first (x(), isLast, implicitWidth, ...), I get a slightly different stack, and they all point to saying that "eng" (text line engine?) got deleted at some point.

Here's one stack example:

Interesting find

While trying to strip down to the minimum the code to reproduce the crash, I stumbled upon something interesting.

In my code above, if I created another dummy AppWindow (visible: false), and then 2nd (and 3rd and so on) AppWindow didn't have the crash and everything is fine.

So for some reason, the 1st instanciated AppWindow has a different behavior.

For example:

QQmlComponent component( _engine, "qrc:/Quick/TestWindow.qml" ); // (where visible  is false)
component.create( nullptr );


QQmlComponent component2( _engine, "qrc:/Quick/TestWindow.qml" );
QQuickWindow *w = qobject_cast<QQuickWindow *>( component2.create( nullptr ) );

w->show();

I really don't understand why, but that's what I have observed.