I actually used FLTK for the first iteration; The theming support it had at the time was not sufficient, though, and the i18n story non existent (that was before even 1.0, versions were still labeled by dates).

So, because pervasive theming/branding was a hard requirement, and i18n a softer one (every skin has to only support one locale), i had to abandon FLTK. But FLTK did spoil me by providing seamless flicker free double buffering out of the box, which I insisted on implementing in our gui.

For those of you who don’t remember - Win32 flickered badly on almost all screen updates, window resizes, etc — I think that was generally true of most apps until 2010 or so. FLTK was essentially the only GUI toolkit at the time (1999) that had no flicker out of the box.

There's another way to build a 150KB or less executable today. Mime is less than 90KB, have a look at https://webd.cf/webd/ It's a web file manager.