First, an edge consists of a one-pixel wide line in my case. Otherwise, I have a QWebView-based widget which I use in a window which can either be docked or not. When it's docked, that widget doesn't look good (to me, at least), so I add a full frame around it, this on Windows and Linux while only a top edge on Mac OS X. Then, when the window isn't docked, I have only a top edge on all three platforms. So, that's it, it's purely for aesthetics.
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