I was tracking down a bug and it turned out to be caused when I call
resize() on a window "from the outside" (so it's my program doing it,
rather than the OS doing it via a window drag).  On OS X (with fltk
head), it turns out that each call to resize() causes it to be called
twice, reentrantly.  I tried it on X11 (with fltk at 3bb34), and the
reentrant call doesn't happen, but resize gets called 4 times.

I'm not sure if this is a bug or not, but it is a bit odd, and it did
cause a bug in my program, which wasn't expecting reentrant calls.
I'm guessing it has to do with how the window system might feel the
need to tweak a resize, for instance on OS X it won't let you overlap
the menu bar.  I'm not sure what a more straightforward approach here
would be, perhaps it would be not to call Fl_Window::resize directly,
but to ask the OS to move the window (how though?), and then the
relevant OS-specific driver will then call the resize with the numbers
it approves of.

Here's a short program to demonstrate the behaviour:

#include <iostream>
#include <FL/Fl_Window.H>
#include <FL/Fl_Box.H>
#include <FL/fl_draw.H>
#include <FL/Fl.H>

class Bug : public Fl_Window {
public:
    Bug(int x, int y, int w, int h) : Fl_Window(x, y, w, h), box(0, 0, w, h)
    {
        box.box(FL_FLAT_BOX);
        box.color(FL_WHITE);
    }

    void resize(int x, int y, int w, int h) override {
        static int nest;
        nest++;
        std::cout << "resize " << nest << "\n";
        Fl_Window::resize(x, y, w, h);
        nest--;
    }

    Fl_Box box;
};


Fl_Window *window;

static void
timeout_func(void *unused)
{
    window->resize(0, 0, 200, 200);
}

int main()
{
    Bug win(200, 200, 200, 200);
    window = &win;
    win.show();
    Fl::add_timeout(1, timeout_func, nullptr);
    Fl::run();
    return 0;
}

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