Bug 96917 [implementation] If two editors use the 2.x-compatible API, one or both doesn't work
At least one problem is that 3.0 installs partitioners for every editor that might possibly edit a file, but each editor expects its own partitioner to be installed. Leading to the famous white screen with blue line.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/ClassFileDocumentProvider.java
return true;
			}
			IJavaElementDelta[] subdeltas= delta.getAffectedChildren();
			for (int i= 0; i < subdeltas.length; i++) {
				if (check(input, subdeltas[i]))
					return true;
			}
			if ((delta.getFlags() & IJavaElementDelta.F_SOURCEDETACHED) != 0 ||
				(delta.getFlags() & IJavaElementDelta.F_SOURCEATTACHED) != 0)
			{
				IClassFile file= fInput != null ? fInput.getClassFile() : null;
				IJavaProject project= input != null ? input.getJavaProject() : null;
				boolean isOnClasspath= false;
				if (file != null && project != null)
					isOnClasspath= project.isOnClasspath(file);
				if (isOnClasspath) {
					fireInputChanged(fInput);
					return false;
				} else {
					handleDeleted(fInput);
					return true;
				}
			}
			return false;
		}
	}
	/**
	 * Correcting the visibility of <code>FileSynchronizer</code>.
	 */
	protected class _FileSynchronizer extends FileSynchronizer {