This is nice. I like how there's a switch that flips between design and testing, and how testing has the familiar insomnia view, with environments.

In the design view, I suggest having a way to show only part of the OpenAPI YAML spec, beyond code folding, because the full YAML document is a lot to scroll through. A way to jump from the view on the right side to the definition in the YAML spec is also important. I suggest adding triple dots at the end of each to show a menu, with "reveal in spec" or something like that.

This is very polished for an initial release. It's a better intro to OpenAPI for someone who is completely new to it than the SmartBear one. Congrats!