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
protected void disposeElementInfo(Object element, ElementInfo info) {
		ClassFileInfo classFileInfo= (ClassFileInfo) info;
		if (classFileInfo.fClassFileSynchronizer != null) {
			classFileInfo.fClassFileSynchronizer.uninstall();
			classFileInfo.fClassFileSynchronizer= null;
		}
		super.disposeElementInfo(element, info);
	}
	/*
	 * @see AbstractDocumentProvider#doSaveDocument(IProgressMonitor, Object, IDocument)
	 */
	protected void doSaveDocument(IProgressMonitor monitor, Object element, IDocument document) throws CoreException {
	}
	/*
	 * @see org.eclipse.ui.texteditor.IDocumentProviderExtension3#isSynchronized(java.lang.Object)
	 * @since 3.0
	 */
	public boolean isSynchronized(Object element) {
		Object elementInfo= getElementInfo(element);
		if (elementInfo instanceof ClassFileInfo) {
			IClassFileEditorInput input= (IClassFileEditorInput)element;
			IResource resource;
			try {
				resource= input.getClassFile().getUnderlyingResource();
			} catch (JavaModelException e) {
				return true;
			}
			return resource == null || resource.isSynchronized(IResource.DEPTH_ZERO);
		}
		return false;