The result of evaluation in binding doesn't get properly shown in the 2D form view

Please see the attached video and the project for the detail.

In Screen01.ui.qml, button1 has x and y bound to an evaluation which is based on the root.widthFactor and root.heightFactor, which in turn are calculated at the root.

Since root.width and root.height are Constants.width and Constants.height respectively, the button1 should be located at (19, 194) in 2D view. 
However, it's located at the top left, indicating that its x and y are regarded as 0.