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
SPELLING_ANNOTATION_TYPE.equals(type);
			}
			/*
			 * @see IJavaAnnotation#hasOverlay()
			 */
			public boolean hasOverlay() {
				return false;
			}
			/*
			 * @see org.eclipse.jdt.internal.ui.javaeditor.IJavaAnnotation#getOverlay()
			 */
			public IJavaAnnotation getOverlay() {
				return null;
			}
			/*
			 * @see IJavaAnnotation#addOverlaid(IJavaAnnotation)
			 */
			public void addOverlaid(IJavaAnnotation annotation) {
				if (fOverlaids == null)
					fOverlaids= new ArrayList(1);
				fOverlaids.add(annotation);
			}
			/*
			 * @see IJavaAnnotation#removeOverlaid(IJavaAnnotation)
			 */
			public void removeOverlaid(IJavaAnnotation annotation) {
				if (fOverlaids != null) {
					fOverlaids.remove(annotation);
					if (fOverlaids.size() == 0)
						fOverlaids= null;