pipeline view is cropped on Firefox

Summary

On Firefox the pipeline view is cropped with white banners on either side, cutting access to the last stage's jobs.

Steps to reproduce

Generate a wide enough (5 stages or more) pipeline, and view it in Firefox

Example Project

(If possible, please create an example project here on GitLab.com that exhibits the problematic behavior, and link to it here in the bug report)

(If you are using an older version of GitLab, this will also determine whether the bug is fixed in a more recent version)

What is the current bug behavior?

While there's ample space, the pipeline view is cropped on its right side by a white/empty vertical strip. A similar strip is found on the left side of the pipeline view as well, between it and the sidebar.

What is the expected correct behavior?

There should be no empty strip on the left, and no cropping on the right

Relevant logs and/or screenshots

Results of GitLab environment info

{"version":"12.0.0-ee","revision":"3b13818e833"}