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
CompilationUnitInfo info= (CompilationUnitInfo)fFakeCUInfoMap.get(element);
		if (info == null) {
			ICompilationUnit cu= createFakeCompiltationUnit(element, true);
			if (cu == null)
				return;
			info= new CompilationUnitInfo();
			info.fCopy= cu;
			info.fElement= element;
			info.fModel= new AnnotationModel();
			fFakeCUInfoMap.put(element, info);
		}
		info.fCount++;
	}
	
	/*
	 * @see org.eclipse.ui.editors.text.TextFileDocumentProvider#getAnnotationModel(java.lang.Object)
	 * @since 3.2
	 */
	public IAnnotationModel getAnnotationModel(Object element) {
		IAnnotationModel model= super.getAnnotationModel(element);
		if (model != null)
			return model;
		
		FileInfo info= (FileInfo)fFakeCUInfoMap.get(element);
		if (info != null) {
			if (info.fModel != null)
				return info.fModel;
			if (info.fTextFileBuffer != null)
				return info.fTextFileBuffer.getAnnotationModel();
		}