After more prodding I’ve discovered that including a home link in my top menu seems to disable the ‘masthead-fixed’ attribute in the body div for all but post pages, so I’ve removed that menu item and all seems to work fine now.
Additional information after more poking and prodding – if I deselect the Top Primary Menu (i.e. set the drop-down to –select–) the masthead becomes sticky, but if I set the Top Primary Menu to my main navigation menu it stops being sticky everywhere except post pages.
Same here… Stats appear fine on JetPack site stats page, but dashboard stats panel just shows “unable to get your stats” message.