Letterboxing & pillarboxing issue

[wesort]

I've been using Fitvids successfully on projects for years, but I've found an issue with Vimeo embeds either letterboxing (black lines above & below) or pillarboxing (black lines left & right) on certain videos. This appears at first, but will then disappear if the viewport is resized or rotated.

• Example: http://theo.wetest.co.uk/work/option-1
• JS code use direct from master and I've renamed #thing-with-videos to .media-container (those are the only code changes)
• Tested with all other JS removed and tried unfloating the containers
• This is on a static site and the issue is present on the 2 devices (Chromebook Pixel & Nexus 5X) I've used to test.

Screenshot of initial page load showing issue:

Screenshot after viewport resize: