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
ExternalClassFileEditorInput external= null;
			if (input instanceof ExternalClassFileEditorInput)
				external= (ExternalClassFileEditorInput) input;
			if (external != null) {
				try {
					refreshFile(external.getFile());
				} catch (CoreException x) {
					handleCoreException(x, JavaEditorMessages.ClassFileDocumentProvider_error_createElementInfo);
				}
			}
			IDocument d= createDocument(input);
			IAnnotationModel m= createClassFileAnnotationModel(input);
			if (external != null) {
				ClassFileInfo info= new ClassFileInfo(d, m,  (_FileSynchronizer) null);
				info.fModificationStamp= computeModificationStamp(external.getFile());
				info.fEncoding= getPersistedEncoding(element);
				return info;
			} else if (input instanceof InternalClassFileEditorInput) {
				ClassFileSynchronizer s= new ClassFileSynchronizer(input);
				s.install();
				ClassFileInfo info= new ClassFileInfo(d, m, s);
				info.fEncoding= getPersistedEncoding(element);
				return info;
			}
		}
		return null;
	}
	/*
	 * @see FileDocumentProvider#disposeElementInfo(Object, ElementInfo)
	 */