I noticed that the constructors for Viewport and TabbedButtonBar call setInterceptsMouseClicks(false, true), and I wonder why.
In my image thumbnail browser, I size the image display sheets to accomodate a variable number of images, using Viewports to scroll them. When images are few, a sheet can be smaller than its Viewport. To catch drags outside the sheet, I made the Viewport a drag target. It took me quite a while to find out that I had to re-enable the mouse interception for this to work.