When switching to fullscreen on a 5K iMac, the ratio changes, and the viewer gets stretched horizontally, making everything smaller and wider, thus unusable. The viewer seems cropped on its right side too (the model is not centered anymore, and the background is cropped, the UI is still well placed though...) Tested on Safari and Chrome with the same result.
Thanks for the very detailed report, helped a lot.
Did a bit of investigation, seems there is a 4096 "webgl screensize" limit on chrome, but good news is that it will be soon removed ( see https://codereview.chromium.org/1040153002 ) so next Chrome version or next Chrome beta version shouldn't have the limitation. (so it might be 2 Chrome version, with 6 weeks between version )
Safari must have inherited of the same limit (safari was derived from chrome code dev code), but developpement is more closed, so no way to predict or know
Meanwhile, here's some way to "fix" it on your side: - firefox seems to work here, so using firefox might fix it for you: https://www.mozilla.org/firefox/new/ - if you can, use of urls parameters might help solve it for you (minus a small quality degradation perhaps) like trying to tweak models urls with "?overrideDevicePixelRatio=0.75" (try the higher ratio possible before it breaks). Bonus is that rendering is way faster and if people look at screen from afar, it won't be noticed.
Thanks, using "?overrideDevicePixelRatio=0.99" solves the problem ! The quality degradation is acceptable. It's great because the default viewer looks really small on such a screen (maybe something like the "theater mode" from youtube would be a nice feature for sketchfab ?)