When using an aspect ratio other than 16:9, background videos will be stretched/squished to fit, making them look weird.
Comparison
P2CE:
Portal 2 (ignore the custom loading icon lol):
Portal 2 fixes this by having separate non-widescreen versions of each background image. These are slightly different from cropped 16:9 versions, with the vignette on the sides narrowed and the framing adjusted slightly in some cases, so the panorama loading screen should support these and Portal 2's versions should be provided with its campaign.
For custom campaigns, since most modern displays are 16:9, most people will probably end up finding this annoying and either lazily crop down 16:9 versions (possibly incorrectly), forget to update them from earlier versions (Mel did this), or not include them at all. Thus, automatic cropping should still be supported and used if no separate 4:3 background images are provided.
This also raises some questions about potential further options you could support for custom campaigns. There should definitely be an option for whether to stretch or crop the backgrounds to fit the screen - for backgrounds that have details around the edges (like vignettes or borders), it may be preferable to have slight distortion on "slightly off" aspect ratios like 16:10 rather than having those details get cut off. Besides that, maybe you could support having separate background images for ultrawide? Or the ability to just specify additional backgrounds for any aspect ratio and it'd pick whatever's closest? Multiple layers with different sizing behavior??? Maybe this is going a bit overboard, but there's definitely a fair bit more options that could be provided here.
When using an aspect ratio other than 16:9, background videos will be stretched/squished to fit, making them look weird.
Comparison
P2CE:Portal 2 (ignore the custom loading icon lol):
Portal 2 fixes this by having separate non-widescreen versions of each background image. These are slightly different from cropped 16:9 versions, with the vignette on the sides narrowed and the framing adjusted slightly in some cases, so the panorama loading screen should support these and Portal 2's versions should be provided with its campaign.
For custom campaigns, since most modern displays are 16:9, most people will probably end up finding this annoying and either lazily crop down 16:9 versions (possibly incorrectly), forget to update them from earlier versions (Mel did this), or not include them at all. Thus, automatic cropping should still be supported and used if no separate 4:3 background images are provided.
This also raises some questions about potential further options you could support for custom campaigns. There should definitely be an option for whether to stretch or crop the backgrounds to fit the screen - for backgrounds that have details around the edges (like vignettes or borders), it may be preferable to have slight distortion on "slightly off" aspect ratios like 16:10 rather than having those details get cut off. Besides that, maybe you could support having separate background images for ultrawide? Or the ability to just specify additional backgrounds for any aspect ratio and it'd pick whatever's closest? Multiple layers with different sizing behavior??? Maybe this is going a bit overboard, but there's definitely a fair bit more options that could be provided here.