Image captured is not relative to scrolled content on health.graphics

[davehunt]

When taking a screenshot of an area outside of the initial viewport on health.graphics, the area captured appears to be relative to the top of the page and doesn't take into account the scroll height.

Steps to reproduce:

1. Visit https://health.graphics/android/tp6m
2. Scroll to the bottom of the page, so the YouTube results are shown
3. Attempt to capture a screenshot of one of the YouTube results

Expected: Results are captured by the screenshot.

Actual: Scroll back to the top of the page, you'll find a capture of the correct dimensions, but not taking the scroll height into account: