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.
All elements that are containers, except menus.
For IupHbox, IupVbox and IupGridBox it consider the MARGIN attribute as a decoration.
For IupDialog, IupBackgroundBox, IupScrollBox and IupGLCanvasBox is only available after the control is mapped.
For IupDialog is NOT read-only, and it will re-define RASTERSIZE by adding the decorations to the actual Client size. (Since 3.3)
For IupSplit returns the total area available for the two children.
SIZE, RASTERSIZE, CLIENTOFFSET