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
* @see IBuffer#getText(int, int)
	 */
	public String getText(int offset, int length) {
		try {
			return fDocument.get(offset, length);
		} catch (BadLocationException x) {
			throw new ArrayIndexOutOfBoundsException();
		}
	}
	/*
	 * @see IBuffer#getUnderlyingResource()
	 */
	public IResource getUnderlyingResource() {
		return fFile;
	}
	/*
	 * @see IBuffer#hasUnsavedChanges()
	 */
	public boolean hasUnsavedChanges() {
		return fTextFileBuffer != null ? fTextFileBuffer.isDirty() : false;
	}
	/*
	 * @see IBuffer#isClosed()
	 */
	public boolean isClosed() {
		return fDocument == null;
	}
	/*
	 * @see IBuffer#isReadOnly()
	 */