Can you elaborate? The last time I looked into this (including a little now) showed that, yes, you can create multiple windows, but there are not good solutions for communicating between the windows. Electron's own documentation shows two, options between local storage and Electron's IPC mechanism. From what I've read, IPC is not good for more complex multiple window apps. So one needs to use a local network communication method.
https://www.electronjs.org/docs/latest/faq#how-to-share-data...
If this is all the case, I would hardly consider it full support.
The only reason Electron exists is to enable Javascript developers do desktop apps without learning other languages and frameworks. Electron is not a solution for desktop development, it's a solution for lazy developers.