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.text.tests/src/org/eclipse/jdt/text/tests/performance/SynchronizedLineDifferInitializationTest.java
private static final Class THIS= SynchronizedLineDifferInitializationTest.class;
	public static Test suite() {
		return new PerformanceTestSetup(new TestSuite(THIS));
	}
	/*
	 * @see org.eclipse.jdt.text.tests.performance.DocumentLineDifferInitializationText#createDocument(java.lang.String)
	 */
	protected IDocument createDocument(String contents) {
		PartiallySynchronizedDocument document= new PartiallySynchronizedDocument();
		document.set(contents);
		return document;
	}
}