There are multiple issues & bugs as far as I can tell:
Position: fixed top and bottom toolbar start to drift as you scroll.
editing in text boxes (inputs are better, textareas much worse) causes conflicts as it turns off fixed positions and sometimes they are there although not visible.
side menus often get cut - a fixed position scrollable side navbar typically gets cut off right above the URL line, but the background continues to be translucent all the way down to the bottom of the screen. This really looks aweful.
top and bottom backgrounds randomly become solid (instead of translucent) to try and compensate for the position:fixed items.
Overall, I have to say this is the buggiest release of Safari for quite a few years now...