I have a 2560×1080 monitor and some web video players, e.g. https://www.channel4.com/now/C4 zoom the video in fullscreen mode so that not all of the video is viewable. This happens on channel4.com but NOT BBC iPlayer so it seems to be related to the sites themselves – likely JS/CSS relating to HTML5 video or flash.
I am aware of a number of chrome extensions which claim to resolve video issues with ultrawide 21:9 monitors but these focus on removing letterboxing which is not the problem I have. I am expecting black bars either side of the image so that the aspect ratio remains in proportion. What seems to be happening is that the picture is being zoomed along the x axis without taking the ultrawide aspect ratio into account.
I’m running Chrome on MacOS 10.15.
Please note this question is not about how to crop black bars.