Assuming that a good unit testing approach is to have one test class per source class (e.g. Strings.java is tested in StringsTest.java), and considering that this is already supported in NetBeans with commands like "Navigate/Go to Test-Tested Class", it would be nice to have the test class embedded in the source class tab of the editor.

As result, every class visualized in the editor would have three views: "Source", "History" and "Test", all in the same tab of the editor. Currently, whenever a class is moved/renamed, the test class has to be moved/renamed manually too. I spend a reasonably amount of time putting together the source class tab with the test class tab in the editor and keeping them synchronized.

With this feature, NetBeans would automatically manage the test classes in a way that they would be always synchronized with the name/location of the source class. "Navigate/Go to Test-Tested Class" would only change the view, not the tab.