The busy indicator (implementations of `IAjaxIndicatorAware`) appears to persist after the AJAX request finishes. To be precise, It looks like it's not being properly cleaned up, as subsequent requests continue to add new indicators to the page. This looks to be new in Wicket 9/9.1, as I didn't observe the behavior in the past (Wicket 7/8) out of the box.
I've seen this occur with uses of `IndicatingAjaxLink.java` along with ModalWindow/ModalDialog, but also for use cases of the vanilla `AjaxIndicatorAppender` behavior attached to a component.
I've attached a small quickstart project which should hopefully make this easy to reproduce. It should suffice to start the project and click any of the two links present.