Improve Fl_Window::hotspot() when diverse screens have diverse scale factors.

src/Fl_Window_hotspot.cxx

@@ -23,6 +23,8 @@ void Fl_Window::hotspot(int X, int Y, int offscreen) {
 
   // Update the screen position based on the mouse position.
   Fl::get_mouse(mx,my);
+  // put the window on the mouse-containing screen
+  this->screen_num(Fl::screen_num(mx, my));
   X = mx-X; Y = my-Y;
 
   // If offscreen is 0 (the default), make sure that the window