The problem exists in all browsers, but it might be more noticeable in some of them. If you have very large images that take multiple seconds to load on any device, you should notice the problem on any browser. It should happen anytime you have images in the ~10 posts prior to the one you opened the permalink of if the images don't fully load in the few hundred milliseconds it takes Flarum to load the single page app, because as soon as the single page app is loaded Flarum applies the scroll position based on the height of the images it knows at that time, which often is very few of them. The page then jumps when the browser finishes loading the missing images.
I actually made an attempt at solving this through my paid Rich Embeds extension https://discuss.flarum.org/d/30516-rich-embeds . If you enable the "Rich Images" feature, the extension will automatically retrieve various meta information from posted images, and if it can calculate the width/height, that value will be applied to the image at page load, which should prevent the jump issue. I added this feature very recently so any feedback is welcome!
