I was actually using Helium for a long time. It is a awesome app as well, offering a different set of features IMHO. A sneak peak of features offered by Fenêt

I was actually using Helium for a long time. It is a awesome app as well, offering a different set of features IMHO. A sneak peak of features offered by Fenêtre. - Browser extension - Code highlight - Markdown Viewer - Click-through and See-through as well as opacity control (Helium has opacity too) - Lots of settings to control defaults behaviors. - Mute window - Clipboard access - Customizable hotkeys - Polished experience for some website (Youtube, Vimeo, Twitch, Github to name a few...) - Access to recent opened items - Local files support (video, image, text files...) - Collapsed view Hope you'll give it a shot !