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 IDocument getDocument() {
		return fDocument;
	}
	/*
	 * @see IBuffer#addBufferChangedListener(IBufferChangedListener)
	 */
	public void addBufferChangedListener(IBufferChangedListener listener) {
		Assert.isNotNull(listener);
		if (!fBufferListeners.contains(listener))
			fBufferListeners.add(listener);
	}
	/*
	 * @see IBuffer#removeBufferChangedListener(IBufferChangedListener)
	 */
	public void removeBufferChangedListener(IBufferChangedListener listener) {
		Assert.isNotNull(listener);
		fBufferListeners.remove(listener);
	}
	/*
	 * @see IBuffer#append(char[])
	 */
	public void append(char[] text) {
		append(new String(text));
	}
	/*
	 * @see IBuffer#append(String)
	 */
	public void append(String text) {
		if (DEBUG_LINE_DELIMITERS) {
			validateLineDelimiters(text);