Fix and add test

Author: Andrew Hall

src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer/Mapping/RazorMapToDocumentRangesEndpoint.cs

@@ -72,7 +72,7 @@ public Uri GetTextDocumentIdentifier(RazorMapToDocumentRangesParams request)
             }
 
             ranges[i] = originalRange;
-            spans[i] = originalRange.ToRazorTextSpan(generatedDocument.Text);
+            spans[i] = originalRange.ToRazorTextSpan(codeDocument.Source.Text);
         }
 
         return new RazorMapToDocumentRangesResponse()

src/Razor/test/Microsoft.AspNetCore.Razor.LanguageServer.Test/Mapping/RazorMapToDocumentRangesEndpointTest.cs

@@ -190,6 +190,44 @@ public async Task Handle_MapToDocumentRanges_Razor()
         Assert.Null(response);
     }
 
+    [Fact]
+    public async Task Handle_MapToDocumentRanges_CSharp_LargeFile()
+    {
+        // Arrange
+        var csharpSource = """
+            class C
+            {
+                public void M()
+                {
+                    var currentTime = DateTime.Now;
+                }
+            }
+            """;
+        var documentPath = new Uri("C:/path/to/document.cshtml");
+        var codeDocument = CreateCodeDocumentWithCSharpProjection(
+            razorSource: "<p>@DateTime.Now</p>",
+            projectedCSharpSource: csharpSource,
+            sourceMappings: [new SourceMapping(new SourceSpan(4, 12), new SourceSpan(66, 12))]);
+        var documentContext = CreateDocumentContext(documentPath, codeDocument);
+        var languageEndpoint = new RazorMapToDocumentRangesEndpoint(_documentMappingService);
+        var request = new RazorMapToDocumentRangesParams()
+        {
+            Kind = RazorLanguageKind.CSharp,
+            ProjectedRanges = [LspFactory.CreateSingleLineRange(line: 4, character: 26, length: 12)],
+            RazorDocumentUri = documentPath,
+        };
+        var expectedRange = LspFactory.CreateSingleLineRange(line: 0, character: 4, length: 12);
+
+        var requestContext = CreateRazorRequestContext(documentContext);
+
+        // Act
+        var response = await languageEndpoint.HandleRequestAsync(request, requestContext, DisposalToken);
+
+        // Assert
+        Assert.NotNull(response);
+        Assert.Equal(expectedRange, response.Ranges[0]);
+    }
+
     private static RazorCodeDocument CreateCodeDocumentWithCSharpProjection(string razorSource, string projectedCSharpSource, ImmutableArray<SourceMapping> sourceMappings)
     {
         var codeDocument = CreateCodeDocument(razorSource, tagHelpers: []);