As the screen shot shows - the sidebarPanel uses only about 50% of the area it covers. This also won't change if I take out 'fileInput' (it only extends horizontally as much as space is available).
The UI code is totally boiler plate which is why I don't think there is much point in posting it. The effect stays the same even when I take out all of the UI input fields. As a matter fact the width of the sidebarPanel is apparently calculated independent of its content but depends on a fixed ratio.
I would like to adjust the size or sidebar-/mainPanel-ratio manually. How could I achieve that?