my favourite environment has always been Open Interface by Neuron Data.
They made an expert system toolkit on Mac in the early 90s and after successfully introducing it to the market, they faced the usual 'cross-platform' dilemma.
But different than most they really nailed it by a common graphic base and GUI toolkit that kept the look and feel of the respective OS while program functionality was retained.
You could even switch the look at runtime instantly from (say) Mac to Motif or Win-3.
While developed for inhouse use originally, it became a product named Open Interface, later versions called Interface Elements (dunno those).
Have a peek in this Discovery Channe feature, lots of classic Macs on the scene...
https://www.youtube.com/watch?v=tSjPNXwcGjAthe interesting part is the 2nd half from about min 5.00 on
The company went through multiple merges and aquisitions and most traces practically vanished from the web.
Found this browsable copy of the original manual:
https://archive.org/stream/TNM_Neuron_Data_Open_Interface_technical_overview_20171017_0142#page/n1/mode/1upTbh it's the most well thought out oo-programming approach without having to deal with an oo-language syntax.
It's plain ansi-C and basically all needed doc is right in the header file comments - short, precise, clear.