Returns the client area size of a container. It is the space available for positioning and sizing children, see the Layout Guide. It is the container Current size excluding the decorations (if any).
"widthxheight", where width and height are integer values corresponding to the horizontal and vertical size, respectively, in pixels. If both values are 0 then "0x0" is returned.
All elements that are containers, except menus.
(*) For IupDialog is NOT read-only, and it will re-define RASTERSIZE by adding the decorations to the given Client size. (Since 3.3)
For IupHbox, IupVbox and IupGridBox it consider the MARGIN attribute as a decoration.
For IupSplit returns the total area available for the two children.