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If the scrollable div ends up as the first-encountered scrollframe, it gets a big displayport that is just over 5000px CSS pixels tall (on GeckoView). When we check for images being "nearly visible" we expand that displayport further by a pref-controlled multiplier, which ends up doubling the displayport height to over 10000 CSS pixels. So for the image to actually get detected as outside the "nearly visible rect" it needs to be positioned further down in the iframe. This test was previously passing on GeckoView because the scrollable div was not getting detected as the first-encountered scrollframe; the root scrollframe for the top-level document was the one getting the displayport even though it didn't really need it. A later patch in this bug changes the first-encountered scrollframe, causing this test to fail without this patch. Differential Revision: https://phabricator.services.mozilla.com/D82777