IMGUI draws within a window, it does not create new windows. FLTK (like other GUI libraries) automatically uses native file dialogs. Drag and drop means you can

IMGUI draws within a window, it does not create new windows. FLTK (like other GUI libraries) automatically uses native file dialogs. Drag and drop means you can drag files into the program, drag strings out of the program, etc. as well as copy and paste in and out of the program. IMGUI is great at what it does, but it does not have native file dialogs or drag and drop. I don't think it interfaces with the OS at all.