Which preference controls whether multiple selection is available to Package Explorer in Eclipse Juno?

Which preference controls whether multiple selection is available to
Package Explorer in Eclipse Juno?

In previous versions of Eclipse, if I selected two files (or directories)
in Package Explorer, I could then right click and use Compare With -> Each
Other.
Since moving to Juno, I have found that my workspaces now only allow
either a single file or a contiguous range of file to be selected. In
other words, Click...Shift+Click does what I expect, but
Click...Ctrl+Click doesn't.
Since this works fine for colleagues working with exactly the same version
of eclipse (we all module load the same Eclipse deployment) I have to
assume that it is a preference of some kind.
Unfortunately, after having gone through the preferences page with a fine
tooth comb, I can't see anything which might control this behaviour.
So, can anyone think of a reason why multi-selection may be failing or
disables in my explorer, but not that of some of my colleagues?