The width property defines the Widget's width.
If the value is positive, it simply defines the Widget's width.
If the value is negative, the Widget's width is defined by (resolution_x - abs(width)).
Click the "flip" button to switch between these two modes.
The width property can be given in absolute pixels or in percent and can be negative or positive. +100% don't mean the total screen width, but total screen height * 4 / 3 to avoid scaling problems between 4:3er and 16:10er resolutions. Use negative percent widths to achieve this. The editor will do this automatically when dragging with the mouse. Hence the value could have these forms:
Click the "flip" button to convert between positive and negative representation (preserving the absolute value) and click the "px" or "%" button to convert between pixels and percent (preserving the absolute value).