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/CompilationUnitDocumentProvider.java
private IProblem fProblem;
			private Image fImage;
			private boolean fQuickFixImagesInitialized= false;
			private int fLayer= IAnnotationAccessExtension.DEFAULT_LAYER;
			public ProblemAnnotation(IProblem problem, ICompilationUnit cu) {
				fProblem= problem;
				fCompilationUnit= cu;
				if (JavaSpellingReconcileStrategy.SPELLING_PROBLEM_ID == fProblem.getID()) {
					setType(SPELLING_ANNOTATION_TYPE);
					fLayer= WARNING_LAYER;
				} else if (IProblem.Task == fProblem.getID()) {
					setType(JavaMarkerAnnotation.TASK_ANNOTATION_TYPE);
					fLayer= TASK_LAYER;
				} else if (fProblem.isWarning()) {
					setType(JavaMarkerAnnotation.WARNING_ANNOTATION_TYPE);
					fLayer= WARNING_LAYER;
				} else if (fProblem.isError()) {
					setType(JavaMarkerAnnotation.ERROR_ANNOTATION_TYPE);
					fLayer= ERROR_LAYER;
				} else {
					setType(JavaMarkerAnnotation.INFO_ANNOTATION_TYPE);
					fLayer= INFO_LAYER;
				}
			}
			/*
			 * @see org.eclipse.jface.text.source.IAnnotationPresentation#getLayer()
			 */
			public int getLayer() {
				return fLayer;
			}