Hold down the command key while clicking on two commits with several files that differ. One of those "commits" must be the working copy.

The list of affected files is generated, along with the text "Displaying all changes between XXXX and working copy." However, upon choosing any file in the list, the diffs do not update; only the last file's differences are ever shown.

Note how, in the attached screen shot, even though "amherst_preview.js" is selected, the differences for style-common.css are still being shown in the right hand pane.

If the differences are generated for two actual commits (not the working copy), the feature works as expected.