fix: topmost line group was not painted

- the topmost line group was not painted in some cases
- re-introduce a decrement lost in previous rework

src/gui/UBBackgroundRuling.cpp

@@ -486,6 +486,8 @@ void UBBackgroundRuling::draw(QPainter* painter, const QRectF& rect, double grid
             {
                 std::swap(minIndex, maxIndex);
             }
+
+            --minIndex;
         }
 
         const auto distFrom = distance(originLine, stackFrom);