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
try {
			if (fTextFileBuffer != null)
				fTextFileBuffer.commit(progress, force);
		} catch (CoreException e) {
			throw new JavaModelException(e);
		}
	}
	/*
	 * @see IBuffer#setContents(char[])
	 */
	public void setContents(char[] contents) {
		setContents(new String(contents));
	}
	/*
	 * @see IBuffer#setContents(String)
	 */
	public void setContents(String contents) {
		int oldLength= fDocument.getLength();
		if (contents == null) {
			if (oldLength != 0)
				fSetCmd.set(""); 
		} else {
			
			if (DEBUG_LINE_DELIMITERS) {
				validateLineDelimiters(contents);
			}
			if (!contents.equals(fDocument.get()))
				fSetCmd.set(contents);
		}
	}