FLTK application shows window behind it as foreground color

Question

No matter the widget and using a new, from-source build of FLTK 1.3.9 on Centos8, I get a large amount of transparency on the foreground color. It renders fine when placed over a black window or desktop. The same code base works well on my OpenSuse machine.

Here's the codebase root

Note also, it doesn't make a difference the settings for OPTION_USE_SYSTEM_LIBJPEG or OPTION_USE_SYSTEM_LIBPNG, nor the color() methods for the base window or widget window.