Ok, tried it out now. Even in the simple codpen it does not make a difference.   One more observation: On iOS 15 you can choose in the Safari setting if you like to have single tabs (url bar on top) or a tab bar (url on bottom). The whole problem only appears to happen when the url bar is on top.   In both scenarios the browser is hiding the bar(s) after a bit of scroll, but it seems only to be a problem when the top and the bottom bar needs to be hidden.