The test creates a frame, sets its preferred size, calls pack() and expects that the getSize() method returns a size not less/greater than the preferred size +/- a margin. The margin value is hard-codded as 15 pixels.

The pack() method actually calls the addNotify() which creates the peer for the frame. When the peer is created it may report more or less correct insets for the frame (this is especially important for the X11 platform). For some window managers/themes the insets may be quite large making the size of the frame greater than the preferred size + 15, which may make the test failing.