appDisplay: Disable built-in mouse scrolling

We want to either handle a scroll event ourselves, or delegate it
to the swipe tracker. What we never want is StScrollView's default
handler that doesn't have any knowledge of pages, so disable it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1560>

js/ui/appDisplay.js

@@ -151,6 +151,7 @@ var BaseAppView = GObject.registerClass({
             x_expand: true,
             y_expand: true,
             reactive: true,
+            enable_mouse_scrolling: false,
         });
         this._scrollView.set_policy(St.PolicyType.EXTERNAL, St.PolicyType.NEVER);