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/DocumentAdapter.java
public boolean isReadOnly() {
		if (fTextFileBuffer != null)
			return !fTextFileBuffer.isCommitable();
		
		IResource resource= getUnderlyingResource();
		if (resource == null)
			return true;
			
		final ResourceAttributes attributes= resource.getResourceAttributes();
		return attributes == null ? false : attributes.isReadOnly();
	}
	/*
	 * @see IBuffer#replace(int, int, char[])
	 */
	public void replace(int position, int length, char[] text) {
		replace(position, length, new String(text));
	}
	/*
	 * @see IBuffer#replace(int, int, String)
	 */
	public void replace(int position, int length, String text) {
		if (DEBUG_LINE_DELIMITERS) {
			validateLineDelimiters(text);
		}
		fReplaceCmd.replace(position, length, text);
	}
	/*
	 * @see IBuffer#save(IProgressMonitor, boolean)
	 */
	public void save(IProgressMonitor progress, boolean force) throws JavaModelException {