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 tap the arrow keys) so window chrome and sticky bars stay out of the stitch, then hit Start. Everything outside your box stays dimmed the whole time.