Right now gesturestart and gesturechange events will cause zooming to occur in Safari by default. For a web application that uses web components and needs to prevent this default behavior from occurring, there is no way to do this without adding an event handler for gesturestart and gesturechange to every custom component.
Please see below for a simple example of this issue:
This should be fixed in iOS 12.2 and macOS 10.14.4 betas.
Thanks Ryosuke, I took a peek at the Safari Technology Preview and it looks fixed there.
(In reply to Ryosuke Niwa from comment #2)
> This should be fixed in iOS 12.2 and macOS 10.14.4 betas.
Great. Thanks for confirming!