in the example i open a page with a scrollable div. This scrollable div contains a green rectangle in the bottom. If you try to make assertion , you obtain a black square. In the second test, i scroll down the div before taking screenshot. Scrolling down the div, i obtain the true screenshot.
E.g.
you have a div
this div contains some elements
those elements height is higher than parent div height so your div is scrollable
if you try to assert element inside scrollable area but out of the visible area, you got a black screenshot.
this is the example code
in the example i open a page with a scrollable div. This scrollable div contains a green rectangle in the bottom. If you try to make assertion , you obtain a black square. In the second test, i scroll down the div before taking screenshot. Scrolling down the div, i obtain the true screenshot.