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I can't speak for CodeApprove, but Reviewable has file grouping capabilities so users can groups files based on anything they want (using a javascript function to do it)

CodeApprove does not have that feature because we don't currently do any smart code parsing to understand what would make a file "related". I think Viezly (https://viezly.com/) does the best job at that.

I feel like if you want that then you should colocate the files in the same folder.

Convention and culture. Hence - I wish that I could.