This is both on Win 7 (32bit) and Ubuntu 12.04, both using Firefox 16.0.2. When I view the side-by-side diff, and hit ctrl-B to hide the sidebar, the diff-by-diff viewport is not expanded horizontally. When I start the side-by-side diff in a Firefox where the sidebar was already hidden, the right side is not rendered readable either. See attached screenshot for a cleaerr view of what I experienced.
Issue #5372 resolved