GitLab edit file web interface broken at narrower viewports

Summary

The edit file web interface is broken at narrower viewports. (Takes up half the available space and blinks.)

This is a showstopper for our new app, where we embed the GitLab web interface as a simple way for people to edit files in a repository.

Steps to reproduce

Go to an edit URL on a narrow-viewport device like an iPhone.

(The example URL in the screenshots, for example, is: https://source.ind.ie/project/set/edit/master/readme.md)

Expected behaviour

The interface should take up the full width of the viewport, should be fully functional, and should not blink.

Relevant logs and/or screenshots

(The screenshots are from the iOS Simulator but the same behaviour is experienced on actual devices.)

iPhone_6s_-_iPhone_6s___iOS_9.2__13C75__2iPhone_6s_-iPhone_6s___iOS_9.2__13C75

Relevant GitLab Environment Info

System information
System:		Ubuntu 14.04

GitLab information
Version:	8.4.2
Revision:	8d07b81

Possible fixes

@laurakalbag has been working on narrowing the issue down and has updated this thread with her findings, below. We are going to implement a fix this week and would love to contribute that back.

I hope this can be included as part of #13586 (closed)