Capture something taller than the screen as one image. Frame the area, you scroll, Plumón stitches, and a live preview grows so you see it working.
macOS can't capture a whole page on its own, so this grabs frames while you scroll and stitches the overlap. You frame the exact scroll area first (drag the edges or arrow-nudge) so window chrome and sticky bars stay out of the stitch, then hit Start. Everything outside your box stays dimmed the whole time.