Some webpages implement lazy loading design.
These kinds of pages basically load content only when the page is being scrolled to the point where the content is needed.
To capture a full-page screenshot of a page, Eyes asks the page for its length and captures the page according to this length.
In pages that implement lazy loading, the length changes as the page is scrolled, due to more content loading.
As a result of this behavior, Eyes is capturing X pixels in length where the actual length of the page is X+Y and the resulted screenshot is shorter then it should be.
The solution is to make sure that the page is fully loaded with all of its content before taking the screenshot.
There is more than one way to perform this and some ways work better than the others.
One way to do this is to scroll the page directly to the footer by using the scrollIntoView() command.
This option does not always trigger the loading of all the content and might not be good enough.
A better solution is to scroll the page one screen at a time to make sure that all the content is being loaded.
Here is one example in C# that implements this solution:
public static void ScrollToTriggerLazyLoading(RemoteWebDriver driver)
long posA = 0;
long posB = 0;
long scrollPos = 0;
posA = (long)js.ExecuteScript("return window.scrollY;");
scrollPos += 300;
js.ExecuteScript("window.scrollTo(0, " + scrollPos + ");");
posB = (long)js.ExecuteScript("return window.scrollY;");
} while (posA != posB);