Fixed
iOS 15 Safari's new address-bar-at-the-bottom triggers resize events way more often than previous versions, causing Snap Slider's update function to blow up any scroll that isn't perfectly horizontal (i.e., all scrolls).
This PR hotfixes the issue by replacing our window resize event handler with a ResizeObserver (in all major browsers—but not Internet Explorer, which gracefully ignores updates).
Now, instead of force-updating slider positions on every window resize event, we only update sliders when their container resizes (much better!).
For more details, see #22.