Curiously some Reels videos don't have this white bar. Nor do "Story" (i.e. the "we stole this from Snapchat" ephemeral content) videos.
Also it's possible to add videos as a "post" (the OG data model), and they're not reels, and these don't seem to have scrolling capability.
Gotta love the "Let's just throw a bunch of crap onto this thing" architectural design. I remember when Reels (the part that is full of reposted TikTok videos and girls in skimpy clothes advertising their OnlyFans) was called IGTV...