Render tree is accessed and modified during Text bounds calculation and other TextHelper operations. These operations can be invoked outside of pulses and so the render tree modifications in them are problematic for the future concurrent rendering.

Here is example of a problematic TextHelper method:

@Override public BaseBounds computeBounds(BaseBounds bounds, BaseTransform tx) {
    Scene.impl_setAllowPGAccess(true);
    NGText ng = (NGText) text.impl_getPGNode();
    text.impl_syncPGNodeDirect();
    Scene.impl_setAllowPGAccess(false);
    return ng.computeContentBounds(bounds, tx);
}

One possible solution was prototyped here:
http://javame-linux.czech.sun.com/r/1085/

Diff:
http://javame-linux.czech.sun.com/r/1085/diff/1-3/