I have two OS X development machines, a beefy Mac Pro and a Mac Book Pro (MBP).
I recently repaved the MBP and reestablished my development toolchain.
When looking at Documentation on the MBP, I noticed that the "Doc Sets" pane displayed (as expected).
On my Mac Pro, no love. I'm not sure how it disappeared, but after several minutes of clicking around with no luck, I got down to business.
A quick scan of
defaults read com.apple.Xcode revealed a whole bunch of potential settings.
On my MBP I saved off a copy of my defaults and then resized the Doc Sets pane.
Another snapshot and a diff revealed a property with the geometry for the Doc Sets window.
quant:jschilli$ defaults read com.apple.Xcode "NSSplitView Subview Frames DocSetsAndSearchResultsSplitView" ( "0.000000, 0.000000, 0.000000, 1249.000000, NO", "0.000000, 0.000000, 1869.000000, 1249.000000, NO" )
That's an awful lot of zeros - and very different from the MBP.
I closed XCode on the Mac Pro, and deleted the key.
defaults delete com.apple.Xcode "NSSplitView Subview Frames DocSetsAndSearchResultsSplitView"
Fired up XCode and low and behold - Doc Sets now display again.