I was using Nitro this morning and looking at photos both from my Photos Library as well as from the File System. I’m not sure what I might have done, but after re-launching Nitro to show only my Photos Library, I can’t scroll through the thumbnails any more. The sidebar is shown, but it now covers some of the thumbnails. Hiding the sidebar allows me to scroll through the thumbnails again, but if I show the sidebar, it covers a portion of my thumbnails, and again, I can’t scroll through the thumbnails. I’m pretty sure I’ve always been able to see all my thumbnails and have been able to scroll through them with the sidebar shown on the side. I’m using Nitro 2025.2 (77) and OS 15.3.1 (Sequoia). Nitro has been working fine up until now. I also saw the weird black line reported in a recent post and it shows up when viewing a single image and corresponds to the black line right below the menu items “Quick Filter” and the search window. I don’t know if that helps you or not, but I thought I’d mention it. I can’t figure out how to reproduce it though.
It sounds like your sidebar is unlocked. Check the toolbar to see the state of the sidebar. If it’s unlocked, then it covers the thumbnails until you click on a folder / album. Then it should hide.
I believe the black line is a macOS bug, but I have not been able to reproduce it to confirm that. If you look at your toolbar, is it also shifted to the right, leaving a gap at the upper left? That would be an OS bug, but I need to figure out a way to work around it.