> Using a web browser as your native app's rendering engine is what all the cool kids are doing these days, so in a way they were just 15 years ahead of the curve.

As others have said parts of explorer were using mshtml for the user interface. However it wasn't just Microsoft. Third party vendors used mshtml as a component to render their UI! You can find samples around still, e.g . https://www.codeproject.com/Articles/6845/Implementing-snap-...

Most memorably, Norton Antivirus 2003 (I think) was one such application. The entire UI was in fact written in HTML with lots of callbacks into c++ code.

So in many ways... Electron isn't all that new of an idea :)