Fl_Group | +----Fl_Tile
The Fl_Tile class lets you resize the children by dragging the border between them:
For the tiling to work correctly, the children of an Fl_Tile must cover the entire area of the widget, but not overlap. This means that all children must touch each other at their edges, and no gaps can't be left inside the Fl_Tile.
Fl_Tile does not normailly draw any graphics of its own. The "borders" which can be seen in the snapshot above are actually part of the children. Their boxtypes have been set to FL_DOWN_BOX creating the impression of "ridges" where the boxes touch. What you see are actually two adjacent FL_DOWN_BOX's drawn next to each other. All neighboring widgets share the same edge - the widget's thick borders make it appear as though the widgets aren't actually touching, but they are. If the edges of adjacent widgets do not touch, then it will be impossible to drag the corresponding edges.
Fl_Tile allows objects to be resized to zero dimensions. To prevent this you can use the resizable() to limit where corners can be dragged to.
Even though objects can be resized to zero sizes, they must initially have non-zero sizes so the Fl_Tile can figure out their layout. If desired, call position() after creating the children but before displaying the window to set the borders where you want.