Hello,
ImageGlass is an excellent program that I'm glad to have discovered. I think it is nearly perfect as a light-weight, functional and customizable photo viewer. There is room for some improvements, though, and one of those would be the ability to customize button functionality a bit more.
For example, in the same way that we can set the middle mouse button and scroll-wheel to do whatever action is most suitable for us (thankfully! So much better than Win 10's image viewer), I'd also like to be able to set Double Left-Click to toggle Fullscreen mode (instead of snapping the selected image to 100% zoom, which is currently what it seems to do).
Also, my mouse has 8 buttons on it, and I'd like to be able to set them to perform additional actions. Particularly, I'd like to be able to set Mouse Button 4 and Mouse Button 5 to be Previous Image and Next Image so that I can scroll through a folder of images without moving the mouse around.