Press and hold the shift key while launching Safari.
- Safari menu : Clear History : all history : [ Clear History ].
- This removes all history and cookies in the browser.
- Safari Preferences : Advanced : [√] Show Develop menu in menu bar
- Necessary before you can clear browser cache
- Press option+command+e to clear browser cache
- Quit Safari
Launch Safari again normally, and try the website again.
All extensions in Safari would be shown in Safari Preferences : Extensions panel, and only can get there in Safari 12 by first running an application designed specifically to install them there. So no issue for you.
What do you mean by active (scrollbar?) with hot links (like Menu) is missing? What is the link to the website so I can see what you are referring too?