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Fl_Flex (shameless plug!) is now officially in upstream FLTK since 1.4 for a "flexbox style" layout manager.
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InfluxDB
Purpose built for real-time analytics at any scale. InfluxDB Platform is powered by columnar analytics, optimized for cost-efficient storage, and built with open data standards.
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Fl_Flow (another shameless plug!) is available for a slightly more novel layout system. This one I feel will work better with C if wrapped.
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libui
Simple and portable (but not inflexible) GUI library in C that uses the native GUI technologies of each platform it supports.
What about https://github.com/andlabs/libui
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I think good option would be nuklear it is a single header lib
NOTE:
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Hence, a higher number means a more popular project.