This feels a bit gross to me.  Here's another alternative:
* Remove the `createdByPanelManager` argument
* _Always_ dispatch an event when `$parent[0] && $parent.is(".content")` is true (e.g. `"editorAreaPanelAdded"`)
* PanelManager would listen for this, and use a guard flag to detect whether or not the event occurred due to one of its own calls to `makeResizable()` or not.  (And if not, PanelManager would be the one to print a deprecation warning).

It wasn't clear to me that would feel much/any cleaner than how it is here, but I could be persuaded...