I had a look at some RiscOS screenshots before writing the last reply. 3.0 didn't have this visual pattern, late 3.x did, but 5.x did. I'm not sure exactly when it showed up, the early ones were all flat.
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No no, I didn't mean to imply the 3D shading _came_ from RISC OS.
What I was saying is that the team lead of the very small group that basically invented the concept of the dock/taskbar reckoned NeXT nicked the idea from them.
My general point being that large UI changes can come from one person having a brainwave -- and have.
The 3D shading, though, I don't know of prior art.
Motif licensed the window furniture and controls from Microsoft, very early on (Windows 2 era), but this is forgotten now.