Place the Visibility Detector element inside the group or Repeating Group cell and you can use in workflow actions Visibility Detector enters screen or Visibility Detector exits screen.
Also you can use Visibility Detector state if it's visible or not.

To detect another element with certain ID make sure "Expose the option to add an ID attribute to HTML elements" is enabled in your app settings in General section and you can set ID of needed element in the bottom of the element properties.

You can leave ID field default value as "id" to detect Visibility Detector itself.

Also, you can set timeout to delay action (useful to show some content in repeating group lately). For example, you can use "current cell index * 100" as timeout and condition to show you main content group in RG when Visibility Detector in this cell is visible so content will be showed later. Be sure to put Visibility Detector outside of hidden group.

Please check plugin demo page:
https://testliveversion.bubbleapps.io/version-test/visibility

and demo editor:
https://bubble.io/page?type=page&name=visibility&version=live&id=testliveversion&tab=tabs-1