The scrollbars in UWP WebView and in Edge have different default behavior leading to many emails to my team. (Everything I talk about here is for the EdgeHtml based WebView and Edge browser and does not apply to the Chromium based Edge browser and WebView2).
There is a Edge only -ms-overflow-style CSS property that controls scroll behavior. We have a different default for this in the WebView as compared to the Edge browser. If you want the appearance of the scrollbar in the WebView to match the browser then you must explicitly set that CSS property. The Edge browser default is scrollbar which gives us a Windows desktop styled non-auto-hiding scrollbar. The WebView default is -ms-autohiding-scrollbar which gives a sort of compromise between desktop and UWP app scrollbar behavior. In this configuration it is auto-hiding. When used with the mouse you'll get Windows desktop styled scrollbars and when used with touch you'll get the UWP styled scrollbars.
Since WebViews are intended to be used in apps this style is the default in order to better match the app's scrollbars. However this difference between the browser and WebView has led to confusion.
Here’s an -ms-overflow-style JSFiddle showing the difference between the two styles. Try it in the Edge browser and in WebView. An easy way to try it in the Edge WebView is using the JavaScript Browser.
