Thanks for the continued guidance!
I had suspected as much from BF_refuse_window.
Shortly after posting I converted the HWND line to what you have mentioned just now, but it didn’t make a difference. I will obviously keep it like this now.
I’ll try to set the origin on the WindowProperties like you suggested and perhaps wait on a FLTK window creation event to open the Panda window.
Something to be noted: I’ve begun to dabble with wxWidgets instead of FLTK, I’ll see if there’s a difference between the two.