It hapens also in the Demo. I'm on El Capitan, latest master.
When I resize the Demo window, random garbage is displayed instead of the browser:
Update: this issue is also related to App Transport Security: "App Transport Security has blocked a cleartext HTTP (http://) resource load since it is insecure. Temporary exceptions can be configured via your app's Info.plist file.". Why there's garbage? I don't know. I flipped through the various demo parts before making this screenshot. So it's probably somesing else