Thanks for your feedback.
You are right, the blue scrollbar a global progress indicator just like your browser main scrollbar on Window. But on Safari on Mac the behavior isn't exactly the same and the browser scrollbar isn't displayed except when you scroll. This is our thinking to keep both visible. We think the top scrollbar is a nice style touch and it's better to make sure all platform sees it. But thanks, this will certainly make us think more!
About scrollbar that can appear in Immersive panel, we don't like that either and recommend to keep your panel short so that they use only a few sentence and will not require scrollbar. A technique for that is to duplicate the view and split your content over multiple panel, you can see that in action in the following "How To story" https://nation.maps.arcgis.com/apps/Cascade/index.html?appid=7a0c165e7b404073b686f95ef98d6241 . We know that's not always possible so we will look into this.
