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
};
			cu= cu.getWorkingCopy(woc, null, getProgressMonitor());
			
			if (setContents) {
				int READER_CHUNK_SIZE= 2048;
				int BUFFER_SIZE= 8 * READER_CHUNK_SIZE;
				Reader in= new BufferedReader(new InputStreamReader(storage.getContents()));
				StringBuffer buffer= new StringBuffer(BUFFER_SIZE);
				char[] readBuffer= new char[READER_CHUNK_SIZE];
				int n;
				try {
					n= in.read(readBuffer);
					while (n > 0) {
						buffer.append(readBuffer, 0, n);
						n= in.read(readBuffer);
					}
				} catch (IOException e) {
					JavaPlugin.log(e);
				}
				cu.getBuffer().setContents(buffer.toString());
				cu.reconcile(AST.JLS3, false, null, getProgressMonitor());
			}
			return cu;
		} catch (CoreException ex) {
			return null;
		}
	}
    private void setUpSynchronization(CompilationUnitInfo cuInfo) {
        IDocument document= cuInfo.fTextFileBuffer.getDocument();
        IAnnotationModel model= cuInfo.fModel;