I've been trying to chase down a rendering bug in 
ToolbarToggleButton (it shows up with a border on the
first paint, but uses proper rollover borders after).
I added some logging code to ToolbarToggleButton.

In the debugger window there is a set of toggle buttons to
show and hide windows.  Whenever you show or hide a window,
the toolbar controlling the windows that are showing is
re-created.  While I don't have measurements of its 
performance impact, this can't be efficient - the set of
available views has not changed, only the state that
the toolbar buttons should represent. It probably
increases the time it takes to update the views.

See 
org.netbeans.modules.debugger.support.nodes.ToolbarView.setV
iews()