In Beta 2 there seems to be a change to the "View->Tool Windows->File Tabs" dialog and its sizing. By default this wasn't ported from my SE V18 settings so I had to manually turn this on. I went to the menu item and enabled it. It started out as a floating window. I drag it to dock under my editor window yet above the "Search Results/Preview/etc" windows. This is the way I did them prior as well. But the File Tabs window width is smaller than my editor window size. This cuts off some of the file names and makes me scroll earlier than I want to.
See attached image.
I was unable to figure out how to widen the tool bar so that it was as wide as my editor window. At one point I couldn't modify the height of the window but that somehow unlocked for me but I'm not sure how.