How does the built-in ScrollBehaviour actually work? https://router.vuejs.org/guide/advanced/scroll-behavior

Do I need to push the current scroll position somehow? Because I have implemented this and it didn't work. I even added a delayed scroll behaviour that returns Promises instead. With these, it works on some browsers only at the very first back-navigation, with others on evey second attempt, and with browsers on iOS never.
So i console-logged the savedPosition and it contains in those failing cases only {left: 0, top: 0}.

  const router = createRouter({
    scrollBehavior(to, from, savedPosition) {
      if (savedPosition) {
        return new Promise((resolve, reject) => {
          setTimeout(() => {
            console.log('Saved Position:', savedPosition)
            resolve({ left: 0, top: savedPosition.top, behavior: 'smooth' });
          }, 800)
        })
      } else {
        return new Promise((resolve, reject) => {
          setTimeout(() => {
            resolve({ top: 0, behavior: 'smooth' });
          }, 800)
        })
      }
    },

Where must I store the saved position (savedPosition) so, that the built-in scroll behaviour has the valid value instead of top: 0?

Here are my findings regarding different browsers:

• Google Chrome Desktop: works mostly always, sometimes not
• Google Chrome on iOS: don't work at all
• Safari Desktop Mac: don't work at all. savedPosition is always top: 0
• Safari iOS: don't work at all. savedPosition is always top: 0

Here are some images from the console, how it looks like in failing cases and in working cases:

Sometimes, on Chrome, the ScrollPosition has a valid value. Sometimes only 0.
On Safari, it will always be 0.