My Gmail Fluid app keeps logging out when I switch windows

I've upgraded to Safari 4 beta. Using GMail inside Safari it occasionally gets a cookie problem which causes a "Bad Request Error 400". So I switched to Fluid, hoping it would avoid that problem, but now I have a new one. If I leave the app (for instance, to visit a web page that it just opened up in Safari), then it logs me out while the app is in the background and I have to type in my username and password again.