public boolean fullCoverageStatusChanged(boolean hasFullCoverageNow) {
        boolean previousState = hasFullCoverage;
        hasFullCoverage = hasFullCoverageNow;
        return previousState != hasFullCoverageNow;
    }
Returns whether this group has too few documents per node to expect it to be balanced
container-search/src/main/java/com/yahoo/search/dispatch/searchcluster/Group.java