What does work:
Browsing the master branch and then switching to view a folder as it stands in a different branch
/projects/widget/repository/C?utf8=✓&rev=Test

What doesn’t work:
Switch to a (non-master) branch then browse into a folder.
/projects/widget/repository/revisions/Test/C

The URLs for the two scenarios should be the same but they are radically different. Updated to the latest release (4.0.11) to check if this has been fixed but it hasn’t been.