@Override
        public void advanceCharactersBackwards() {
            doWithConfig(sourceConfig -> {
                final long maxLines = sourceConfig.getMaxHexDumpLines();
                final long maxBytesPerFetch = HexDump.MAX_BYTES_PER_LINE * maxLines;
                final long newByteOffsetFrom = Math.max(
                        0,
                        receivedSourceLocation.getDataRange().getByteOffsetFrom() - maxBytesPerFetch);
                setDataRange(DataRange.fromByteOffset(newByteOffsetFrom));
            });
        }
Used for navigating HexDump data which has to navigate using byte offsets,
not char offsets.
stroom-core-client/src/main/java/stroom/data/client/presenter/SourcePresenter.java