macOS Player Progress Bar Issue?

What does that light gray on the progress bar mean? And why is that a bit above the main progress bar, it really is annoying to see it’s not aligned.

Just a guess but I think it shows how much of the content is buffered from the source where the movie is loaded from

Yup, it’s the buffer size. I think that there’s a fix coming in 7.3 for some issues with the progress bar on the Mac version but I don’t know if the offset is part of it. That thread is here.