Well, I was leaning to the Selenium API enhancement direction and make use of that. The Selenium Firefox has: get_full_page_screenshot_as_file which should work out of the box with Selenium 4. I am against doing this with the implementation provided in the first comment.

This functionality would be nice, but I see various problems in the proposed implementation:

• Not sure would the newly opened browser see the exact same page than the original one. The new browser would also miss possible custom configuration that the original had.
• 12 second sleep.

I just wanted to let you know that I have amended the above script so that is useful for me in my project.

This is the solution I am currently using:

def capture_fullpage_screenshot(url):
    """ Creating a Full-Page screenshot.

    Use the already running selenium driver and create a screenshot.
    It works when running headless.
    Aim of this library is to be used for debugging.
    """
    seleniumlib = BuiltIn().get_library_instance('SeleniumLibrary')

    current_window = seleniumlib.driver.get_window_size()

    seleniumlib.driver.get(url)

    time.sleep(4)

    height = seleniumlib.driver.execute_script("return document.body.scrollHeight")
    seleniumlib.driver.set_window_size(1920, height)

    ROOT_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
    screenshotpath = os.path.join(os.path.sep, ROOT_DIR, 'reports' + os.sep)

    now = datetime.now()
    timestamp = now.strftime("%H%M%S")

    BuiltIn().log_to_console(screenshotpath + "full_page_screenshot_{}.png".format(timestamp))
    seleniumlib.driver.save_screenshot(
            screenshotpath + "full_page_screenshot_{}.png".format(timestamp)
    )
    time.sleep(4)

    seleniumlib.driver.set_window_size(current_window["width"], current_window["height"])

It is still not sufficient to be integrated to the Selenium Library I am afraid.
I wouldn't know how to remove the sleeps, for example, as the time is needed for the resizing of the window.
Having said that, I should also say that this version makes use of the already open session.

Knowing when to webapp is ready is actually quite difficult problem and therefore removing the sleep may require application specific logic. I would still lean to use the Selenium API and make an implementation based on that, even when it works only with Firefox. But it's good that you got your keyword improved and I bet you learned lot of things.