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A while ago, while working on a Plone 3.3 site (which had been migrated from Plone 3.1.7), we realized that comparing different versions of an object on the "history" tab didn't show anything. So we went and looked at the CMF Diff Tool (portal_diff) in the zmi and saw that it was empty when we would have thought that at least the basic Plone types would be handled.

We tested on a vanilla Plone 3.3 site and the CMF Diff Tool was NOT empty.

We assumed somehow something made these settings disappear from the CMF Diff Tool and fixed it with an upgrade step:

from Products.CMFCore.utils import getToolByName


def rebuild_portal_diff(self):
    """For the given list of types, add the to the portal_diff tool
    as Compound Diff's
    """
    to_add = [
        'Document',
        'Event',
        'Favorite',
        'File',
        'Folder',
        'Image',
        'Link',
        'News Item',
    ]
    pdiff = getToolByName(self, 'portal_diff')
    for element in to_add:
        pdiff.manage_addDiffField(element, 'any', 'Compound Diff for AT types')

Today, one of our clients reported the same issue with a Plone 3.1.7 site. So the first reflex was to create a vanilla Plone 3.1.7 site to see how it was behaving: the CMF Diff Tool was empty. Then we migrated this vanilla site to Plone 3.3.5 and the CMF Diff Tool still was empty.

In the end, if you happen to have a site that was created before Plone 3.3, you'll have to add each type in the CMF Diff Tool if you want to be able to see differences between versions on the "history" tab of an object.

In the meantime, I reported the issue at https://dev.plone.org/plone/ticket/11389.