values are automatically updated when the combo box unit changes
This means that you can flip between units and things like
the existing width and height are converted immediately to the
new unit
When adding a new item to a layout, if the user just does a single
click of the mouse (vs a click and drag to set the new item
position/size), then a dialog is shown asking to user for
the new item size/position/reference point.
This allows easy creation of items with an exact position/size
and is common behavior in most proper DTP/illustration apps.
