Developers typically expect APIs like MouseEvent.clientX/Y and getBoundingClientRect to use the same co-ordinate system as position:fixed (what we've been calling "layout viewport co-ordinates"). In Safari's new visual viewport model these APIs return visual co-ordinates instead, resulting in some site bugs under pinch-zoom. As a simple example see https://rbyers.github.io/inputCoords.html - when pinch-zooming in the red box is wrong. In addition, some sites don't expect scroll offsets to change when panning the pinch zoom viewport, or that changing the scroll position will reset the position of the pinch zoom viewport. For example on apple.com if you zoom in and click the search button in the top bar the viewport doesn't get centered correctly on the search field because there's a window.scrollTo(0,0) call. Here's some example sites that are broken in Safari today: https://docs.google.com/document/d/1jxWEzwOT2XgSsg5k2a_3D7PR0bXnB4bL9rATW-Z6egY/edit Talking with smfr@ he agrees with the principle that pinch-zoom should be completely transparent to the application by default. Developers generally are not thinking about the impact of pinch-zoom, and pinch-zoom is primarily a UA/accessibility feature. He says he'd like to change WebKit to make all these APIs use layout viewport co-ordinates instead of visual viewport ones (i.e. to match Chrome's behavior with the chrome://flags/#inert-visual-viewport flag enabled - see https://crbug.com/489206)