Bare Bones Software's venerable
BBEdit text editor uses a similar custom of a hollow or filled diamond character with the same meaning (although it doesn't appear in the title bar). That's how I came to be familiar with it.
Although, I vaguely remember the diamond symbol also appearing in classic Mac OS apps, where it meant the same thing. Of course, BBEdit was born in the classic era, so maybe they just carried it over. The diamond symbol seems to be less common in Mac OS X, perhaps because the OS provides the "dirty" indicator in the red close button in the upper left of app windows, as jsturtridge pointed out.
I'm not familiar with the history of OmniOutliner though. I didn't pick up my first copy until relatively recently (OmniOutliner 2 sometime in 2004, I think). My impression is that OmniOutliner wasn't available on the Mac platform prior to OS X.
Two other things that have always troubled me about OmniOutliner's use of that diamond:
- I haven't done much AppleScripting of OmniOutliner, but when I did, it always seemed a hassle to identify a specific window because the diamond is part of the name. Maybe there's a better way. Like I said, I didn't dig very deep.
- I've always wondered why hoisting/unhoisting a section is considered a content change (solid diamond) rather than a view change (hollow diamond). That doesn't seem right to me.
-Dennis