Why is it that you can see it if you screenshot it?

The steps I took were: Right click on the image and selected copy. Paste it into Paint (which showed the logo).

I then saved it as a PNG (still showing the logo in Windows image viewer) and uploaded to Imgur. I assume pasting it into paint compressed the colors?

I saved it as a PNG and uploaded it. I don't know the technical details behind this but I'm assuming Paint compressed the colors.

PNGs are saved as sRGB by default, and so the differences aren't remapped away by the OS like they would be if the PNG was saved with a wider color space.

wait so if you screenshotted that.. what ends up saved in the PNG? Does the image clipboard now automatically have 2 versions of any screenshot; a wide colour version and some sort of normalised 8 bit version?

I'm looking at this from a macbook pro 2015 so I assume no wide colour support (?), yet I see the logo. Which conversion, at which step, caused that effect?

2015 MacBooks are not Display P3. What browser are you using, as that is incorrect behaviour.

The clipboard here is probably copying the image buffer in the untransformed state, and pasting it into MSPaint would dump those values incorrectly.

I see the same thing in both Firefox and Safari, on a non-P3 display: the original image is solid red, but the copied image shows the logo. So it looks like a conversion did actually happen at some point.

The transform to display the "solid red" block is actually the post-transformed data. The clipboard (rightfully) is carrying the original source buffer.

The issue would be the mechanics of the copy (attaching metadata) and pasting (interpreting the metadata in conjunction with the source image buffer.)

So technically, no, there's no conversion on the source data, it's the representation in the pasted context that's the issue.