Use this method to add controls to a list of controls that the WindowSplitter will internally manage. Once you have told the WindowSplitter about controls that should be affected by the splitter movement then the WindowSplitter will take care of resizing and moving them when the WindowSplitter moves.
Using this method can in most cases eliminate the need to do size calculations in the SplitterMoved event.
Don't use this method for a TabPanel or PagePanel as there is a bug in REALbasic which lets REALbasic brute force destruct those controls when the window closes no mater what the reference counter says.
Use AddUnmanagedControl instead when adding a TabPanel or PagePanel control.