Event listener on page scroll

I was also looking for such an event, but here is the best  I found. Scripts don't work well with events, and to create plugin is too expensive...

File name: displayedPage.jsx

#targetengine displayedPage
/**
    Put this file into "startup scripts" and reload ID.
    When you change active page the message about page number appears...
    However, the page becomes active only if a selection appears on it. 
    Unfortunately simple scrolling does not trigger an events.
*/
function displayedPage(){
    
    app.addEventListener('afterOpen', function(event){
        if (event.parent instanceof LayoutWindow) {
            
            var aDoc = app.activeDocument;
            var lWindow = event.parent;
            
            alert('Document name: ' + app.activeDocument.name);
            
            lWindow.addEventListener('afterAttributeChanged', function(wevent){
                var l_window = wevent.parent;
                alert("Displayed Page: " + l_window.activePage.name);
            });
                       
        }
    });

    /**
        This function is required, otherwise, the script will work even after deletion! 
    */
    app.addEventListener('beforeQuit', function(){
        app.eventListeners.everyItem().remove();
    });
}

displayedPage();

Thanks @Laubender 

Page number shown at the bottom panel is reliable, but my client wants to see it in bigger view. So thinking to show it in the panel.

Can you please suggest which Constants/Event s I can attach with the layout window to get the active window page number.