I recently bought Duet Display which allows you to use your (compatible) iOS device as an external display on your Mac.
It’s quite nice to use TaskPaper on my iPad when I’m working in Yosemite’s full / split screen mode. You can even use touch. While it doesn’t work well to drag and drop items, it’s quite nice to be able to view / switch between projects and saved searches without leaving my main work screen.
There’s one glitch: the sidebar turns black when TaskPaper is inactive, but you can still read the project names and as soon as the window is activated, everything is back to normal.
Maybe this is useful for you, too.