1. Resize your browser window so that it doesn't cover the entire desktop (ie is not maximized)
2. Search users and select a user
The user record is opened in the third, right-most pane. The three panes cover the window from left to right.
The user record is opened in the third pane. To the right of the third is a blank area that stretches all the way from the bottom of the page to the top-most Apps header. The width of the blank area seems to differ depending on how reduced the size of the window is when you open the user record.
This only happens if the window size is reduced before you open the user record. Resizing it once a user record is already open produces the expected result.