Fix GetMinMergedCell for cells spanning all columns and multiple rows

src/foundation/src/MigraDoc/src/MigraDoc.Rendering/Rendering/TableRenderer.cs

@@ -903,6 +903,37 @@ Cell GetMinMergedCell(int row)
                     return cell;
                 }
             }
+
+            // Fallback: all cells in this row begin multi-row spans (MergeDown > 0),
+            // so no cell satisfies `resultRowIndex + MergeDown == row`.
+            // This occurs when a single cell spans all columns and more than one row
+            // (e.g. MergeRight = columnCount - 1, MergeDown >= 1).
+            // Find the cell with the minimum MergeDown instead —
+            // this matches the behaviour of GetMinMergedCellOriginal.
+            int minMergeDown = int.MaxValue;
+            Cell? minCell = null;
+            for (int idx = 0; idx < clsCount; idx++)
+            {
+                var cell = resultRow[idx];
+                if (!_mergedCells.Contains(cell))
+                    continue;
+                if (cell.MergeDown < minMergeDown)
+                {
+                    minMergeDown = cell.MergeDown;
+                    minCell = cell;
+                    if (minMergeDown == 0)
+                        break; // Can't get better.
+                }
+            }
+            if (minCell is not null)
+            {
+#if DEBUG
+                Debug.Assert(originalResult.Row!.Index + originalResult.MergeDown ==
+                             minCell.Row!.Index + minCell.MergeDown);
+#endif
+                return minCell;
+            }
+
             throw new InvalidOperationException("GetMinMergedCell: Unexpected problem #1");
         }