Screenshot a full page when an inner div scrolls, not body

In app shells where the layout is a fixed-height flex column (sidebar + main with overflow-y-auto), document.body.scrollHeight equals innerHeight, so scroll down N and full-page screenshot flags both no-op and you silently capture only the first viewport. Detect the real scroller first with an eval that filters all elements for scrollHeight > clientHeight + 20 && clientHeight > 200, then drive it directly by setting element.scrollTop and screenshotting between steps. Resizing the viewport does not help either when the browser window is fixed.