Prior to September 1, 2023, this took a default value of 100 and was interpreted as pixels, unless the value was 0 and it was the last item in the list, in which case it would use the remaining space in the window.
It now allows you to provide your own value for WidthUnits.
Additionally, the default value used to be an arbitrary value of 100. It is now 0, to take advantage of the automatic proportional calculator in the new version. If you want the old behavior, pass 100, StatusBar.Part.WidthUnits.DeviceIndependentPixels.