So, sadly, I realized that this highly-retweeted Flexbox Inspector screenshot shows two visual centering issues: https://twitter.com/FirefoxDevTools/status/1080578449276719104 1. Sibling selector should be centered in the pane, ignoring the space that the back button takes up. In this screenshot, with these dimensions, it basically needs to move 25 px to the left. 2. The diagram should be centered in the pane, ignoring if one of its labels hang over the side. (Ideally the labels wouldn't hang over the side.) In this screenshot it basically needs to move about 20px to the right.