Gets the current extent of the editor's graphical representation.
The result is only valid when the editor is displayed (see Basic Organization). For text% objects, calling this method may force the recalculation of location information if a maximum width is set for the editor, even if the editor currently has delayed refreshing (see refresh-delayed?).
The w box is filled with the editor's width, unless w is #f. The h box is filled with the editor's height, unless h is #f.