private @NotNull String assertMessage() {
    String startOffset = "startOffset: " + mySegmentStartOffset;
    String endOffset   = "endOffset: " + mySegmentEndOffset;
    String foldIndex   = "foldIndex: " + myCurrentFoldRegionIndex;
    String foldCount   = "foldCount: " + (myRegions != null ? myRegions.length : "null");
    String foldRegion  = "fold" + ((myRegions != null && myCurrentFoldRegionIndex < myRegions.length) ? myRegions[myCurrentFoldRegionIndex].toString() : "");
    String inlayIndex  = "inlayIndex: " + myCurrentInlayIndex;
    String inlayCount  = "inlayCount: " + (myInlays != null ? myInlays.size() : "null");
    String inlay       = "inlay: " + ((myInlays != null && myCurrentInlayIndex < myInlays.size()) ? myInlays.get(myCurrentInlayIndex).toString() : "");
    String textLength  = "textLength: " + myDocument.getTextLength();
    return String.join(
      ", ",
      startOffset,
      endOffset,
      foldIndex,
      foldCount,
      foldRegion,
      inlayIndex,
      inlayCount,
      inlay,
      textLength
    );
  }
private @NotNull String assertMessage()