fullscreen/api/element-request-fullscreen-same-element.html

<!DOCTYPE html>
<title>Element#requestFullscreen() on the current fullscreen element</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-vendor.js"></script>
<div id="log"></div>
<script>
promise_test(async (t) => {
    t.add_cleanup(() => {
        if (document.fullscreenElement) return document.exitFullscreen();
    });

    // Use the body element as the fullscreen element, because the second
    // test_driver.bless() call needs to insert a button inside of it, which
    // can't be clicked if another element is already fullscreen.
    const target = document.body;

    // First enter fullscreen.
    await test_driver.bless("fullscreen", () => target.requestFullscreen());
    assert_equals(document.fullscreenElement, target, "fullscreen element after first request");

    // Now request fullscreen again, which should be a no-op.
    await test_driver.bless("fullscreen", () => target.requestFullscreen());
    assert_equals(document.fullscreenElement, target, "fullscreen element after second request");
});
</script>