Region.USE_COMPUTED_SIZE is supposed to cause the compute[Min|Pref|Max][Width|Height] to return the region's intrinsic size. Region.USE_PREF_SIZE is supposed to cause the compute[Min|Max][Width|Height] to return the value of computePref[Width|Height]. The compute methods do not follow this.

Example: Region computeMinHeight

    @Override protected double computeMinHeight(double width) {
        return getInsets().getTop() + getInsets().getBottom();
    }

This code should be

    @Override protected double computeMinHeight(double width) {
        if (getMinHeight() == USE_PREF_SIZE) {
             return computePrefHeight(width);
        }
        else if (getMinHeight() == USE_COMPUTED_SIZE || getMinHeight() < 0) {
            return getInsets().getTop() + getInsets().getBottom();
        }
        else {
            return getMinHeight();
        }
    }

This affects any code that overrides these compute methods.