That you get  current_timer->timer == nullptr most probably means that delete_timer()

has been called before repeat_timeout() was called.

This means that the control did not stay continuously inside your callback function,

but was returned to cocoa at some point by your callback, in contradiction to what is required to use repeat_timeout().

No, it actually meant that a stray thread on my side was calling Fl::remove_timeout, breaking both the contract with fltk (of not calling things outside the main thread) and breaking Fl::repeat_timeout.

Thanks, Manolo.  Your advice is always helpful.