Keep layers comma-separated when exporting a multi-layer shorthand

src/ExCSS.Tests/PropertyTests/BackgroundProperty.cs

@@ -736,5 +736,26 @@ public void BackgroundImageDataUrlLegal()
             Assert.True(concrete.HasValue);
             Assert.Equal("url(\"" + url + "\")", concrete.Value);
         }
+
+        [Fact]
+        public void BackgroundMultipleLayersExportCommaSeparatedLonghands()
+        {
+            // Each comma-separated layer must stay its own layer when the shorthand is exported to the
+            // longhands; joining them with whitespace produces one invalid layer instead of two valid ones.
+            var style = ParseDeclarations("background: url(a.png) no-repeat, url(b.png) repeat");
+
+            Assert.Equal("url(\"a.png\"), url(\"b.png\")", style.BackgroundImage);
+            Assert.Equal("no-repeat, repeat", style.BackgroundRepeat);
+        }
+
+        [Fact]
+        public void BackgroundThreeLayersExportCommaSeparatedLonghands()
+        {
+            var style = ParseDeclarations(
+                "background: url(a.png) no-repeat, url(b.png) repeat-x, url(c.png) repeat");
+
+            Assert.Equal("url(\"a.png\"), url(\"b.png\"), url(\"c.png\")", style.BackgroundImage);
+            Assert.Equal("no-repeat, repeat-x, repeat", style.BackgroundRepeat);
+        }
     }
 }

src/ExCSS/ValueConverters/EndListValueConverter.cs

@@ -92,7 +92,11 @@ public TokenValue ExtractFor(string name)
 
                     if (extracted == null) continue;
 
-                    if (tokens.Count > 0) tokens.Add(Token.Whitespace);
+                    // Layers stay comma-separated when exported to a longhand, matching CssText above and
+                    // ListValueConverter's own ExtractFor. Joining with whitespace instead ran the layers
+                    // together: "background: url(a) no-repeat, url(b) repeat" gave a background-repeat of
+                    // "no-repeat repeat", which is a single invalid layer rather than two valid ones.
+                    if (tokens.Count > 0) tokens.Add(Token.Comma);
 
                     tokens.AddRange(extracted);
                 }