ScrollBar does not account for the fact that there may be a top/bottom margin set on the containing control. For example, if the ScrollBar is in a TableView with a top/bottom margin set then it will have a size of less than 1.0 indicating that the scrollbar is needed even though it is not. This does not happen if the ScrollBar is in a ScrollView, it is only an issue if it is in a TableView or Flickable.

Additionally, if the model changes from needing a scrollbar to not then it will still indicate the scrollbar size as being less than 1.0.