The Qt Quick Controls 2 ComboBox list now jumps on creation on Qt 5.15.0.
By jump, I mean that the list position changes just after apparition.
I have a video to show the behavior, I will try to attach it to the ticket.

I have tested it with the Qt Quick Controls 2 Gallery example by adding some more elements to trigger this behavior. The same application does not show this behavior on Qt 5.12.8.

Changes in the Gallery example, ComboBox.qml:

83c83
<             editable: true
---
>             editable: false
88a89,92
>                 ListElement { text: "Lemon" }
>                 ListElement { text: "Orange" }
>                 ListElement { text: "Peach" }
>                 ListElement { text: "Strawberry" }