The attached sample customizes ScrollBar for a ScrollView. The result is weird that both custom ScrollBar and default ScrollBar coexist.

Reason:
Internal Flickable is not yet available at the time of neither default nor custom ScollBar getting set. Related codes:
https://github.com/qt/qtdeclarative/blob/dev/src/quicktemplates/qquickscrollbar.cpp#L1275
https://github.com/qt/qtdeclarative/blob/dev/src/quicktemplates/qquickscrollbar.cpp#L1224
The cleanup codes are never executed for setting a new ScrollBar.

Workaround:
Set custom ScrollBar at a later time. For example, as in the sample, in Main.qml when ScrollView is Completed.

So either we change this behavior or warn user in documentation.