Dragrable panels to resize containers by dragging the space between them

dragable-panels.rkt

#lang racket/gui

(require framework)

(define fr (new frame% [label "gui"] [width 500] [height 500]))
(define p (new panel:vertical-dragable% [parent fr]))
(define pt (new panel:horizontal-dragable% [parent p] [border 1] [style '(border)]))
(define pb (new panel:horizontal-dragable% [parent p] [border 1] [style '(border)]))
(define ptl (new panel% [parent pt] [border 1] [style '(border)]))
(define ptr (new panel% [parent pt] [border 1] [style '(border)]))
(define pbl (new panel% [parent pb] [border 1] [style '(border)]))
(define pbr (new panel% [parent pb] [border 1] [style '(border)]))

(send fr show #t)