On iOS Safari, touch drag events propagate up to the page body, which means the background page remains scrollable even when a popup is open. This doesn't affect other browsers or desktop devices.
What you might notice
When a popup is open on an iOS device in Safari, the background page can still be scrolled
This doesn't occur on desktop browsers or other mobile browsers
Workaround
Make the popup taller than the full screen height — this makes the popup itself scrollable, which prevents users from noticing the background scrolling behind it
Use an opaque overlay behind the popup to hide the background content entirely
This behaviour originates from how Apple propagates touch drag events on iOS, which Bubble can't override. You can read more in the documentation.