Bug 240892

When zooming out on a web page, Safari triggers a "Tab Expose" view as a result of overscroll, and when scrolling backwards or forwards triggers navigation.

I'd expect that, similar to other gestures, calling preventDefault() on the resulting GestureEnd would prevent these system gestures from occurring, but that isn't the case. On iOS, it's a known workaround to preventDefault() on the touchstart event that begins the gesture, but similar workarounds (like preventDefault() on the mousedown) seem to not have the same effect on macOS in Safari from my experimentation.

As an alternate approach, I also tried out overscroll-behavior. The specification for overscroll-behavior (which will be the [new default soon](https://bugs.webkit.org/show_bug.cgi?id=236060) and that I experimented with activating for myself locally via the "Experimental Features") states that "[contain] indicates that the element must not perform non-local boundary default actions such as scroll chaining or navigation." Even when applying "overscroll-behavior: contain" to an element, both of these non-local interactions (swiping backwards, and pinching to zoom) are still triggered.