Very much the same sentiment as Brian. I tried to run the Winforms-On-Mono implementation. The UI was buggy and I sometimes couldn't navigate to pages on the internet normally, though I'm willing to bet there was a setting to change in Fiddler that might've alleviated this. One thing I thought was interesting was that I was using it on a mac connected to one external monitor, and when I dragged Fiddler over from the primary laptop monitor to the external/secondary monitor, the drop down menus ceased to work and swathes of the UI went blank. Like, the drop downs were visible, and the action of clicking would cause the UI to show clicking, but then no menu would appear. This behavior didn't happen on the primary monitor. Thought you'd want to know.
(MBP Retina 13 inch mid 2014 / macOS 10.12 / 2.6 GHz Core i5 / 16 GB 1600 MHz DDR3 / Intel Iris 1536 MB Graphics).
Thanks for your work, Mr. Yovtchev, keep it up.