Shotwell would be perfect, if it weren't for a scrolling bug that makes it really annoying to use.

I like that shotwell has a directory tree on the side that recursively shows the contents of whatever folder I click on. The main viewing area displays thumbnails of all my images. Double clicking on an image displays it using the entire viewing area, with a basic zoom control and ability to go full screen. This UI is perfect for what I want and I can't seem to find any suitable alternatives. On to why I'm not happy with Shotwell --

So my issue with Shotwell is a scrolling bug that makes it really annoying to look through my pictures. I have to hold my trackpoint within a very slim speed for scrolling, and forget about trying to scroll with the touchpad. A far lesser issue; I'd also prefer to get away from GTK stuff and use Qt stuff so I can have a native Oxygen theme instead of an ugly GTK-ported Oxygen theme.

Any picture library program I find that seems promising ends up not being recursive with the selected directory in the tree. That's a vital feature for me.

I'm not looking for an image editor. I'm looking for an advanced viewer to go though my massive and very deep Pictures folder.

Check out Mirage - https://mirageiv.sourceforge.net/

It doesn't show the contents of directories recursively. That's a must have for me.