As title says I'd like to have a fully transparent main FLTK window/frame background. The below code works just fine:

#include <Windows.h>
#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Button.H>
#include <FL/Fl_PNG_Image.H>
#include <FL/FL_Box.H>

// Global pointers for the GUI objects
Fl_Window* mywindow;
Fl_Box* mypicturebox;
Fl_PNG_Image* img;

//int main() {
int WINAPI WinMain(HINSTANCE hThisInst, HINSTANCE hPrevInst, LPSTR str, int nWinMode) {

    mywindow = new Fl_Window(295, 290, "Color changer");
    mywindow->color(FL_WHITE);

    // Load some images to use later
    img = new Fl_PNG_Image("D:/Pobrane_2/flower.png");

    // A box for the image
    mypicturebox = new Fl_Box(0, 0, 295, 290);

    // Give it some initial contents
    mypicturebox->image(img);

    mywindow->box(FL_NO_BOX);

    Fl_Button but(50, 80, 100, 20, "Click Me!");
    but.callback([](Fl_Widget* w, void*) {
        Fl_Widget* p = w->parent();
        mywindow->box(FL_FLAT_BOX);
        mywindow->box(FL_NO_BOX);
        mywindow->set_changed();
        mywindow->redraw();
        p->redraw();
        });

    mywindow->show();
    return Fl::run();
}

enter image description here

...UNTIL the app windows is moved:

enter image description here

Then the all effect just gone wrong. As You can see the window appears NOT to be refreshed/updated because in the transparent background we have old/stale content what was relevant ONLY the time when the app window have been created, that is BEFORE the repositioning.

As can bee seen I've added a button whit some refreshing code on it's on-click event but even after executing that forced refresh it's still NOT working as expected!

Any idea how the code can be corrected to have a valid "behind" background content after the window move?