In the MercurialEclipse plugin repo, select aae72537e493 changeset in the history view *opened on the repository root*, it contains two files in the "changed files" table. Try to right click any one of them => no context menu is shown. Historically those files were in the hg repository root directory, but than they were moved one level below. Because of that I guess the compare action code seems to fail to find the right ancestor path in the project (the files now are "outside" the current project) and so fails to show the menu.
This is also broken for single files history: try to open history for MercurialStatusCache, the context menu in the "changed files" table will mostly be broken for versions before 1.8.2 release tag.
Additionally the double click on the changeset will show "no changes" dialog.
Independently of which file were selected in the changeset, a right click context menu must contain entries like "Compare with current" or "Compare with previous", and lot of other stuff.