Never mind, I figured it out. One of the comments in my inbox has images that are wider than the inbox would like to be. I did not notice this because I'm in Tropospherical Purple, bookkeeper looks to be in Tropospherical Red, and consequently the background color of their cropped screenshots and the background color of my inbox match. Select that comment and suddenly the background in these images is really visible against the purple of the inbox. Collapse that comment and the inbox width problem goes away.