possible fix for fleaflet#1510

Author: ibrierley

lib/src/layer/polyline_layer.dart

@@ -69,10 +69,10 @@ class PolylineLayer extends StatelessWidget {
   final bool saveLayers;
 
   const PolylineLayer({
-    super.key,
-    this.polylines = const [],
-    this.polylineCulling = false,
-    this.saveLayers = false,
+  super.key,
+  this.polylines = const [],
+  this.polylineCulling = false,
+  this.saveLayers = false,
   });
 
   @override
@@ -82,8 +82,8 @@ class PolylineLayer extends StatelessWidget {
 
     final List<Polyline> lines = polylineCulling
         ? polylines.where((p) {
-            return p.boundingBox.isOverlapping(map.bounds);
-          }).toList()
+      return p.boundingBox.isOverlapping(map.bounds);
+    }).toList()
         : polylines;
 
     return CustomPaint(
@@ -141,9 +141,7 @@ class PolylinePainter extends CustomPainter {
     int? lastHash;
 
     void drawPaths() {
-      canvas.drawPath(path, paint);
-      path = ui.Path();
-      paint = Paint();
+      if (saveLayers) canvas.saveLayer(rect, Paint());
 
       if (borderPaint != null) {
         canvas.drawPath(borderPath, borderPaint!);
@@ -156,6 +154,12 @@ class PolylinePainter extends CustomPainter {
         filterPath = ui.Path();
         filterPaint = null;
       }
+
+      canvas.drawPath(path, paint);
+      path = ui.Path();
+      paint = Paint();
+
+      if (saveLayers) canvas.restore();
     }
 
     for (final polyline in polylines) {
@@ -165,7 +169,7 @@ class PolylinePainter extends CustomPainter {
       }
 
       final hash = polyline.renderHashCode;
-      if (lastHash != null && lastHash != hash) {
+      if (saveLayers || (lastHash != null && lastHash != hash)) {
         drawPaths();
       }
       lastHash = hash;
@@ -225,7 +229,6 @@ class PolylinePainter extends CustomPainter {
       final radius = paint.strokeWidth / 2;
       final borderRadius = (borderPaint?.strokeWidth ?? 0) / 2;
 
-      if (saveLayers) canvas.saveLayer(rect, Paint());
       if (isDotted) {
         final spacing = strokeWidth * 1.5;
         if (borderPaint != null && filterPaint != null) {
@@ -240,7 +243,6 @@ class PolylinePainter extends CustomPainter {
         }
         _paintLine(path, offsets);
       }
-      if (saveLayers) canvas.restore();
     }
 
     drawPaths();
@@ -281,16 +283,16 @@ class PolylinePainter extends CustomPainter {
 
   List<double>? _getColorsStop(Polyline polyline) =>
       (polyline.colorsStop != null &&
-              polyline.colorsStop!.length == polyline.gradientColors!.length)
+          polyline.colorsStop!.length == polyline.gradientColors!.length)
           ? polyline.colorsStop
           : _calculateColorsStop(polyline);
 
   List<double> _calculateColorsStop(Polyline polyline) {
     final colorsStopInterval = 1.0 / polyline.gradientColors!.length;
     return polyline.gradientColors!
         .map((gradientColor) =>
-            polyline.gradientColors!.indexOf(gradientColor) *
-            colorsStopInterval)
+    polyline.gradientColors!.indexOf(gradientColor) *
+        colorsStopInterval)
         .toList();
   }
 
@@ -301,4 +303,4 @@ class PolylinePainter extends CustomPainter {
         oldDelegate.polylines.length != polylines.length ||
         oldDelegate.hash != hash;
   }
-}
+}