In an environment where my business partners start all their emails with:
subject: re: xyzxy (and never change the xyzxy over 30 replies),
this addon is truely helpful to find the one message in the sequence of re's that I need.
one question: the body is fetched on hover or pre-fetched on folderdisplay? (= efficiency on really large folders or offline use)?
It's up to you - see the Options/Preferences in Add-ons Manager. The default is to fetch on mouse hover. Although everything is async, fetching on row scroll into view when scrolling is very fast will be cpu intensive like most things in treeview.