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Note in spec that button may be disabled, from commenters: It also seems plausible to me that Show Source may sometimes have to be disabled. I can imagine findbugs for example generating a warning about some inconsistency in the project where there is no specific associated source location, for example if the project contains a .class file with no associated source.
fixed http://core.netbeans.org/source/browse/core/www/proposals/tasklist_ui_spec.htm?r1=1.1&r2=1.2