LineLayout getPrefixLayout() {
    synchronized (myLock) {
      FoldRegion[] topLevelRegions = myEditor.getFoldingModel().fetchTopLevel();
      if (topLevelRegions != null && topLevelRegions.length > 0) {
        FoldRegion firstRegion = topLevelRegions[0];
        if (firstRegion instanceof CustomFoldRegion && firstRegion.getStartOffset() == 0) {
          return null; // prefix is hidden
        }
      }
      return myPrefixLayout;
    }
  }
LineLayout getPrefixLayout()