I have a proposed fix - please let me know if I've analyzed the problem correctly.
The modal dialog tries to set event.ignore=true if the browser is Safari. This Wicket event data structure used to be passed through to Wicket.Drag.mouseDownHandler as the argument e. However, in the switch over to jQuery events the argument (e) passed to mouseDownHandler is no longer Wicket "event". It is now jQuery event data.
There is a test in mouseDownHandler to check for the existence of e.ignore. As this is now jQuery data, it _never_ exists, so Safari is never recognized.
I changed this test to check event.ignore and it appears to work correctly.