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/PartiallySynchronizedDocument.java
private final Object fInternalLockObject= new Object();
    private Object fLockObject;
    
    /*
     * @see org.eclipse.jface.text.ISynchronizable#setLockObject(java.lang.Object)
     */
    public void setLockObject(Object lockObject) {
        fLockObject= lockObject;
    }
    /*
     * @see org.eclipse.jface.text.ISynchronizable#getLockObject()
     */
    public Object getLockObject() {
        return fLockObject == null ? fInternalLockObject : fLockObject;