The UML modules have "border buildup" problems in a lot of places.  I worked
very hard in the rest of NetBeans to fix all such cases - the typical pattern is
adding something to a JScrollPane that supplies its own border, like a
JTextComponent - you get doubled borders.  This causes the UML components to
look sloppily designed  beside the rest of the IDE's components.

Screenshots for some cases attached - it would be worth reviewing all uses of
JScrollPane to make sure this is handled correctly.  Typically the solution is
simply to remove the borders from the scroll pane in question.  Code:

theScrollPane.setBorder (BorderFactory.createEmptyBorder());
theScrollPane.setViewportBorder (BorderFactory.createEmptyBorder());

be sure to set the viewport border as well as the outer border - different look
and feels will favor different borders.  Do *not* use null, use an EmptyBorder -
if you use null, GTK look and feel will treat that as an invitation to supply
its own border, making things ugly again.