A few observations ...

>Maybe CreateBitmap requires the data to be still alive?

The docs don't say that. The docs have been wrong before ...

> fluid.exe!Fl_GDI_Graphics_Driver::create_bitmask(int w, int h, const unsigned char * data) Line 393

The matching line in Github is line 390. Explanation for the discrepancy?

>the bitmap width or height is 0, so "new[w*h]" returns NULL

This is true if FLTK was built against the C std library (not the C++ std library). In which case the library is not C++ standard

conformant [new should not return NULL]; the assumption that delete (null) will be handled might not be valid?

Kevin