main.cpp

/*
 * OS:              Ubuntu 18.04
 * Kernel:          Linux 4.15.0-42-generic
 * Compiler:        GCC 7.3.0
 * Libraries:       gtkmm 3.22.2-2
 * Instructions:    Compile with 'g++ -o test main.cpp `pkg-config --libs --cflags gtkmm-3.0`'
 *                  and run with './test'.
 * Description: Creates window called 'TITLE' of size 'WIDTH' x 'HEIGHT'
 *                  and fills it with RGB color that is specified using 'Color' struct.
 *                  The drawing area onto which is drawn is placed into scrolled window
 *                  so that we can use the scrollbars to navigate the drawing area
 *                  if the window is resized to smaller than initial size.
 *                  The problem is that these scrollbars are not shown.
*/

#include <gtkmm.h>
#include <string>

// window attributes
#define TITLE "Test"
#define WIDTH 640
#define HEIGHT 480

// color to display in RGB format
struct Color
{
    double black = 0.0;
    double green = 0.0;
    double red = 0.0;
};

class DrawingArea : public Gtk::DrawingArea
{
    public:
        // copies dimensions of the window so the whole window is filled
        void copy_dimensions(Gdk::Rectangle& dimensions)
        {
            memcpy(&_dimensions, &dimensions, sizeof(Gdk::Rectangle));
            signal_draw().connect(sigc::mem_fun(*this, &DrawingArea::render));
            size_allocate(_dimensions);
        }

    private:
        Gdk::Rectangle _dimensions;

        // renders color onto the surface
        bool render(const Cairo::RefPtr<Cairo::Context>& cr)
        {
            Color color;
            cr->set_source_rgb(color.black, color.green, color.red);
            cr->rectangle(_dimensions.get_x(), _dimensions.get_y(), _dimensions.get_width(), _dimensions.get_height());
            cr->fill();
            return true;
        }
};

class Window : public Gtk::Window
{
    public:
        Window(const int width, const int height)
        {
            // set dimensions
            _dimensions.set_x(0);
            _dimensions.set_y(0);
            _dimensions.set_width(width);
            _dimensions.set_height(height);

            // set properties of window and its elements
            describe_window();

            // set drawing area
            _drawing_area.copy_dimensions(_dimensions);

            // attach all together
            attach_elements();
        }
    private:
        Gdk::Rectangle _dimensions;
        Gtk::ScrolledWindow _scroll_win;;
        DrawingArea _drawing_area;

        // set major window properties
        void describe_window()
        {
            // window itself
            set_title(TITLE);
            set_default_size(_dimensions.get_width(), _dimensions.get_height());

            // scrolling area
            _scroll_win.set_policy(Gtk::PolicyType::POLICY_AUTOMATIC, Gtk::PolicyType::POLICY_AUTOMATIC);
            _scroll_win.set_overlay_scrolling(true);
            _scroll_win.set_hexpand(true);
            _scroll_win.set_vexpand(true);
        }

        // properly glues all elements together
        void attach_elements()
        {
            _scroll_win.add(_drawing_area);
            add(_scroll_win);
        }
};

int main(int argc, char *argv[])
{
    // create app and run it
    Glib::RefPtr<Gtk::Application> test = Gtk::Application::create(argc, argv, "this.is.just.a.test");
    Window win(WIDTH, HEIGHT);
    win.show_all();
    return test->run(win);
}