I'm no mumblecore slut and found the two previous Duplass Brothers films I'd seen--The Puffy Chair and Cyrus--to be an off-putting cross between Judd Apatow and Henry Jaglom, but Jeff, Who Lives at Home is lovely. Jeff (Jason Segel) is an unemployed, 30-year-old pothead living in the basement of his widowed mom, Sharon (a surprisingly tolerable Susan Sarandon). He's obsessed with the movie Signs, but the filmmakers seem to accept, rather than ridicule, that it is simply the thing that helped him crystallize his fatalistic belief system, and that if his worldview were less limited by circumstances--namely, depression--he would perhaps…