I came across a problem in re-sizing the Taskpaper window when you either hide or show the sidebar. When the sidebar is hidden and Taskpaper is all the way to the left edge of the screen and you select "Show sidebar," the sidebar then "appears" off the screen. To make it visible, you then have to drag the window to the right.
Similarly, if the sidebar is visible and you select the option to hide it, the window size shrinks and the document you're working in stays the same size. It seems like it would be better to keep the window the same size and make the writing area wider when you hide the sidebar.
I was just testing the app out to decide whether to buy it when I came across this problem.