There appear to be two different issues here: 1. **Whether the fixed header is included in the overscroll animation.** It looks to me like Reddit can serve two different page structures: one where the <html> is scrollable, and one where a <div> containing the contents under the header is scrollable. You can tell the difference from the scollbar: if the scrollbar overlaps the header when scrolled to the top, it's the <html>, but if the scrollbar starts underneath the header, it's the <div>. The fixed header is included in the overscroll animation only when the <html> is scrollable. So, while it's strange for Reddit to be switching the page structure like this, the behaviour is expected from the browser's point of view. 2. **Sometimes, overscroll does not work at all if you start scrolled to the top** (but it does if you scroll down and back up). This seems to only happen in the <div> case. This one requires further investigation.
Bug 1709575 Comment 2 Edit History
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There appear to be two different issues here: 1. **Whether the fixed header is included in the overscroll animation.** It looks to me like Reddit can serve two different page structures: one where the <html> is scrollable, and one where a <div> containing the contents under the header is scrollable. You can tell the difference from the scollbar: if the scrollbar overlaps the header when scrolled to the top, it's the <html>, but if the scrollbar starts underneath the header, it's the <div>. The fixed header is included in the overscroll animation only when the <html> is scrollable. While it's strange for Reddit to be switching the page structure like this, the behaviour is expected from the browser's point of view. 2. **Sometimes, overscroll does not work at all if you start scrolled to the top** (but it does if you scroll down and back up). This seems to only happen in the <div> case. This one requires further investigation.