Suppress the “are you sure you want to leave this page” popup in the Delphi TWebbrowser control

Question 1

This message is shown by the underlying MSHTML engine if the web page handles window.onbeforeunload event. Usually, it's there for a reason, to let the user know his/her input hasn't been saved or submitted yet. The prompt suppression script from the answer you linked doesn't work for cases when the page uses addEventListener("beforeonload", handler) or attachEvent("onbeforeunload", handler). I don't think there's a reliable way of doing this, without resorting to low-level Windows hooks.

[UPDATE] The following script (look for "Inject this script") is a hack which aggressively suppresses the page's own handlers for onbeforeunload event, via setInterval. It should work in 99% of cases, but it still leaves a gap for the page to override onbeforeonload right before navigating away.

<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <title></title>
    <script type="text/javascript">
        window.attachEvent("onbeforeunload", function (ev) {
            window.event.returnValue = "onbeforeunload via window.attachEvent()";
        });

        window.addEventListener("beforeunload", function (ev) {
            window.event.returnValue = "onbeforeunload via window.addEventListener()";
        });

        window.onbeforeunload = function (ev) {
            window.event.returnValue = "onbeforeunload via window.onbeforeunload";
        };
    </script>

    <script type="text/javascript">
        //
        // Inject this script
        //
        (function () {
            var onbeforeunloadHandler = function (ev) {
                if (ev) {
                    if (ev.stopPropagation)
                        ev.stopPropagation();
                    if (ev.stopImmediatePropagation)
                        ev.stopImmediatePropagation();
                    ev.returnValue = undefined;
                }
                window.event.returnValue = undefined;
            }

            var handler = null;
            var intervalHandler = function () {
                if (handler)
                    window.detachEvent("onbeforeunload", handler);

                // window.attachEvent works best
                handler = window.attachEvent("onbeforeunload", onbeforeunloadHandler);
                // handler = window.addEventListener("beforeunload", onbeforeunloadHandler);
                // handler = window.onload = onbeforeunloadHandler;
            };

            window.setInterval(intervalHandler, 500);
            intervalHandler();
        })();
    </script>

</head>
<body>
    <a href="http://www.example.com">Go away</a>
</body>
</html>

To inject this script with Delphi, you'd probably need to resort to low-level WebBrowser/MSHTML COM interfaces, like IWebBrowser2, IHTMLDocument2, IHTMLScriptElement, in a very similar way it's done in the linked answer. With some more efforts, the same can also be done via late binding, using IDispatch::GetIDsOfNames and IDispatch::Invoke only. If you're asking for exact Delphi code, I don't have one.

Question 2

The other answer you link to handles the browser's Navigated event. In it, it injects a script element into the page and into each frame on the page. That script assigns a new value to window.alert so that when other code on the page calls it, it does nothing.

Question 3

This code resets the event handler:

var
  WrkIHTMLWindow2: IHTMLWindow2;
  WrkIHTMLWindow2Disp: IHTMLWindow2Disp;
begin
  WrkIHTMLWindow2 := IHTMLDocument2Disp(WrkIWebBrowser2.Document).parentWindow;
  if WrkIHTMLWindow2.QueryInterface(IHTMLWindow2Disp, WrkIHTMLWindow2Disp) = S_OK then
    if not VarIsNull(WrkIHTMLWindow2Disp.onbeforeunload) then
      WrkIHTMLWindow2Disp.onbeforeunload := NULL;
end;