How do I use an NSProgressIndicator as the view of an NSToolbarItem and have it show in the customisation sheet?

Question

I have an NSToolbar set up in my app. It was created in IB. One of the toolbar items uses a spinner-style NSProgressIndicator as its view, the rest are images.

When the toolbar's customization sheet is showing, the spinner does not show in the sheet.

I initially thought that this was because the spinner's -displayedWhenStopped was set to NO. That's not the case: if I set it to YES, the spinner shows in the toolbar when active, but doesn't show in either the toolbar or the sheet when stopped.