See the attached screenshot. When I try to load a side-by-side diff, the right side is actually displayed below the left side. This is with Chrome 25.0.1364.172 on Ubuntu Linux. It works fine in Firefox.
Editing to add :
- Either reset browser zoom to 100%
- Or enable new pull request experience by logging in to bitbucket.org > select your avatar (bottom left corner) > Labs > find and enable the "New pull request experience"