How can I make Side-by-side be the default diff for my GitLab installation or project or profile?
Update February 2016
The issue is now at issue CE 3071 and... was resolved by commit 9fdd605!
A cookie now retain your diff view choice, with GitLab 8.5.0+.
Original answer (February 2015)
That doesn't seem to be possible right now, and can be voted up at the suggestion 7082397
Remember side-by-side diff choice
Right now when I want to review code, I have to choose side-by-side diff each time I open a Merge Request because that choice is not stored/saved anywhere.
Either store my last choice/change of viewer globally somewhere and use it on all diff views from that point on.
Or let me have a config option on my account where I can specify my preferred choice.
The side-by-side diff view was introduced with GitLab 6.4 (Dec. 2013) with commit b51c2c8.