Confusing ScrollView in Qt.Quick.Controls2

[fromis_9]

// Qt 5.15.2
import QtQuick 2.15
import QtQuick.Controls 2.15

ScrollView { id: scrollView
  function scrollToY(y) {
    scrollView.contentItem.contentY = y
    scrollView.contentItem.returnToBounds() // this works
  }
  contentItem.boundsBehavior: Flickable.StopAtBounds 
  // Cannot assign to non-existent property "boundsBehavior"
 
  Pane {
  // ...
  // my contents is in this Pane
  // ...
  }
}

Above code is where I lost.
In ScrollView QML Type docs, there is NO description that what type the contentItem is. List of All Members for ScrollView docs said contentItem is the member inherited from Control, and the type is Item. But my contentItem above code has returnToBounds() method and it works!!
So I assumed that the contentItem's type is maybe Flickable, cause the only type which has returnToBounds() is Flickable, and expect that it also has boundsBehavior property. But it doesnt.
I'm soooo confused. Why the document has no full description about contentItem in ScrollView? What the heck is real contentItem?

[GrecKo]

As far as the QML engine knows, it's an Item. That's why you can't do contentItem.boundsBehavior: Flickable.StopAtBounds. Left-hand expressions have to fully typed and can't dynamically adapt. Right-hand expressions or Javascript expressions can, that's why you can call returnToBounds() in your function.

contentItem is in fact a Flickable but you can use this fact for a declarative binding on it.
Most background are Rectangle but they are declared as Item and since you could change it to be a simple Item or any of its subclasses you can't do background.radius: 8 but you can do background.visible: false because visible is a property from Item.

In you case you could set the boundsBehaviour in a Component.onCompleted handler or with a Binding:

Binding {
    target: scrollView.contentItem
    property: "boundsBehaviour"
    value: Flickable.StopAtBounds
}