I'm using Xcode 26.4 for Swift development on macOS 26.4.1.
It seems that the documentation for the "Foundation" framework has disappeared, possibly after an Xcode update (not the latest one). "Foundation" does not appear in the left-side list in the developer documentation window, and searching for "URLSession", "UndoManager" finds no result.
Autocomplete for these classes works. Quick help on URLSession shows just "inherits from" and "conforms to", but no description and no link to a help page.
I've tried deleting files in ~/Library/Developer so that it rebuilds the index, and also fully removing and reinstalling Xcode and rebooting, but nothing seems to solve it. Is there any way to get the full documentation back in Xcode.