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.