I'm trying to figure out how to get a child widget to conform to the dimensions of it's parent widget exactly. While the code below (modeled after the code from Fl_pack.cxx does a decent job of resizing the child widget, when resizing the window the child and parent can differ a pixel or two in size. This is particularly noticeable when when resizing the window quickly.

It seems the drawing of the child is somehow "lagging behind" the drawing of the parent.

I don't know the language you posted, but it looks like you might be resizing
in the draw() method, that'd explain why you're seeing a lag.

Don't do resizing in draw().
The ONLY thing you should really do in draw() code is draw stuff.

If you want precise resizing, subclass the group widget and override the resize(x,y,w,h)
method, and inside that:

    1) Tell the baseclass to resize(),
    2) Set the child to the size you want
    3) Call init_sizes() last, so the group knows you changed the size of one of its children