Fix HTML completion race when typing too quickly

- There was a race when typing too quickly our synchronization mechanism would try and reduce the work that was done to cancel pre-existing requests. So in the scenario when you were to type `<tr` what would happen is that 3 completion requests would fire:
    1. Triggered completion for `<`
    2. Typing completion for `t`
    3. Typing completion for `r`
    Now when we sped things up we were able to process requests in parallel which meant that we could handle simultaneous requests for `<`, `t` and `r` all at the same time; however, this in turn results in an interesting behavior where if we ask for completion at `r` while `<` and `t` are still active our synchronization mechanism would aggressively cancel the older requests. For completion this is catastrohpic because it would result in a 0 length completion list because HTML does not respect completion requests beyond trigger characters and the beginning of the word.
- To fix this issue I added a new mechanism for synchronization which takes a flag `rejectOnNewerParallelRequest` which states do not reject a synchronization request aggressively if this flag is `true`. Now this doesn't mean the requests never get rejected. If there's a batched document update or document close / open this will still reject the document; it just means that on the requesting of synchronization that older completion requests are not rejected eagerly.
- Added tests to our projection and synchronization stack to account for this
- Ensured that these changes are not breaking changes so marked some bits as virtual.

### Before

![before image](https://i.imgur.com/sZfGaub.gif)

### After

![after image](https://i.imgur.com/5EsJdDm.gif)

Fixes #5743

src/Razor/src/Microsoft.VisualStudio.LanguageServer.ContainedLanguage/DefaultLSPDocumentSynchronizer.cs

@@ -38,6 +38,9 @@ namespace Microsoft.VisualStudio.LanguageServer.ContainedLanguage
         }
 
         public override Task<bool> TrySynchronizeVirtualDocumentAsync(int requiredHostDocumentVersion, VirtualDocumentSnapshot virtualDocument, CancellationToken cancellationToken)
+            => TrySynchronizeVirtualDocumentAsync(requiredHostDocumentVersion, virtualDocument, rejectOnNewerParallelRequest: true, cancellationToken);
+
+        public override Task<bool> TrySynchronizeVirtualDocumentAsync(int requiredHostDocumentVersion, VirtualDocumentSnapshot virtualDocument, bool rejectOnNewerParallelRequest, CancellationToken cancellationToken)
         {
             if (virtualDocument is null)
             {
@@ -59,7 +62,7 @@ namespace Microsoft.VisualStudio.LanguageServer.ContainedLanguage
                 }
 
                 // Currently tracked synchronizing context is not sufficient, need to update a new one.
-                var onSynchronizedTask = documentContext.GetSynchronizationTaskAsync(requiredHostDocumentVersion, cancellationToken);
+                var onSynchronizedTask = documentContext.GetSynchronizationTaskAsync(requiredHostDocumentVersion, rejectOnNewerParallelRequest, cancellationToken);
                 return onSynchronizedTask;
             }
         }
@@ -192,14 +195,15 @@ namespace Microsoft.VisualStudio.LanguageServer.ContainedLanguage
                 }
             }
 
-            public Task<bool> GetSynchronizationTaskAsync(int requiredHostDocumentVersion, CancellationToken cancellationToken)
+            public Task<bool> GetSynchronizationTaskAsync(int requiredHostDocumentVersion, bool rejectOnNewerParallelRequest, CancellationToken cancellationToken)
             {
                 // Cancel any out-of-date existing synchronizing contexts.
 
                 for (var i = _synchronizingContexts.Count - 1; i >= 0; i--)
                 {
                     var context = _synchronizingContexts[i];
-                    if (context.RequiredHostDocumentVersion < requiredHostDocumentVersion)
+                    if (context.RejectOnNewerParallelRequest &&
+                        context.RequiredHostDocumentVersion < requiredHostDocumentVersion)
                     {
                         // All of the existing synchronizations that are older than this version are no longer valid.
                         context.SetSynchronized(result: false);
@@ -207,7 +211,7 @@ namespace Microsoft.VisualStudio.LanguageServer.ContainedLanguage
                     }
                 }
 
-                var synchronizingContext = new DocumentSynchronizingContext(requiredHostDocumentVersion, _synchronizingTimeout, cancellationToken);
+                var synchronizingContext = new DocumentSynchronizingContext(requiredHostDocumentVersion, rejectOnNewerParallelRequest, _synchronizingTimeout, cancellationToken);
                 _synchronizingContexts.Add(synchronizingContext);
                 return synchronizingContext.OnSynchronizedAsync;
             }
@@ -230,10 +234,12 @@ namespace Microsoft.VisualStudio.LanguageServer.ContainedLanguage
 
                 public DocumentSynchronizingContext(
                     int requiredHostDocumentVersion,
+                    bool rejectOnNewerParallelRequest,
                     TimeSpan timeout,
                     CancellationToken requestCancellationToken)
                 {
                     RequiredHostDocumentVersion = requiredHostDocumentVersion;
+                    RejectOnNewerParallelRequest = rejectOnNewerParallelRequest;
                     _onSynchronizedSource = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
                     _cts = CancellationTokenSource.CreateLinkedTokenSource(requestCancellationToken);
 
@@ -243,6 +249,8 @@ namespace Microsoft.VisualStudio.LanguageServer.ContainedLanguage
                     _cts.CancelAfter(timeout);
                 }
 
+                public bool RejectOnNewerParallelRequest { get; }
+
                 public int RequiredHostDocumentVersion { get; }
 
                 public Task<bool> OnSynchronizedAsync => _onSynchronizedSource.Task;

src/Razor/src/Microsoft.VisualStudio.LanguageServer.ContainedLanguage/LSPDocumentSynchronizer.cs

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the MIT license. See License.txt in the project root for license information.
 
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -9,5 +10,21 @@ namespace Microsoft.VisualStudio.LanguageServer.ContainedLanguage
     internal abstract class LSPDocumentSynchronizer : LSPDocumentChangeListener
     {
         public abstract Task<bool> TrySynchronizeVirtualDocumentAsync(int requiredHostDocumentVersion, VirtualDocumentSnapshot virtualDocument, CancellationToken cancellationToken);
+
+        /// <summary>
+        /// Attempts to synchronize a virtual document to a corresponding host document version.
+        /// </summary>
+        /// <param name="requiredHostDocumentVersion">The corresponding host document version required for the generated <paramref name="virtualDocument"/></param>
+        /// <param name="virtualDocument">A generated document to correlate <paramref name="requiredHostDocumentVersion"/>'s for.</param>
+        /// <param name="rejectOnNewerParallelRequest">
+        /// When <c>true</c> if a second synchronization request for the same virtual document comes in with a newer required host document version all pending synchronization requests will be rejected.
+        /// If <c>false</c>, active synchronization requests will be fulfilled as virtul document buffers get updated. Fulfillment could be a rejection or an approval.
+        /// </param>
+        /// <param name="cancellationToken"></param>
+        /// <returns><c>true</c> if we were able to successfully synchronize; <c>false</c> otherwise.</returns>
+        public virtual Task<bool> TrySynchronizeVirtualDocumentAsync(int requiredHostDocumentVersion, VirtualDocumentSnapshot virtualDocument, bool rejectOnNewerParallelRequest, CancellationToken cancellationToken)
+        {
+            throw new NotImplementedException();
+        }
     }
 }

src/Razor/src/Microsoft.VisualStudio.LanguageServerClient.Razor/HtmlCSharp/CompletionHandler.cs

@@ -147,7 +147,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 return null;
             }
 
-            var projectionResult = await _projectionProvider.GetProjectionAsync(
+            var projectionResult = await _projectionProvider.GetProjectionForCompletionAsync(
                 documentSnapshot,
                 request.Position,
                 cancellationToken).ConfigureAwait(false);
@@ -532,7 +532,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
             }
 
             var previousCharacterPosition = new Position(projection.Position.Line, projection.Position.Character - 1);
-            var previousCharacterProjection = await _projectionProvider.GetProjectionAsync(
+            var previousCharacterProjection = await _projectionProvider.GetProjectionForCompletionAsync(
                 documentSnapshot,
                 previousCharacterPosition,
                 cancellationToken).ConfigureAwait(false);

src/Razor/src/Microsoft.VisualStudio.LanguageServerClient.Razor/HtmlCSharp/DefaultLSPProjectionProvider.cs

@@ -61,7 +61,13 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
             _loggerProvider = loggerProvider;
         }
 
-        public override async Task<ProjectionResult?> GetProjectionAsync(LSPDocumentSnapshot documentSnapshot, Position position, CancellationToken cancellationToken)
+        public override Task<ProjectionResult?> GetProjectionAsync(LSPDocumentSnapshot documentSnapshot, Position position, CancellationToken cancellationToken)
+            => GetProjectionCoreAsync(documentSnapshot, position, rejectOnNewerParallelRequest: true, cancellationToken);
+
+        public override Task<ProjectionResult?> GetProjectionForCompletionAsync(LSPDocumentSnapshot documentSnapshot, Position position, CancellationToken cancellationToken)
+            => GetProjectionCoreAsync(documentSnapshot, position, rejectOnNewerParallelRequest: false, cancellationToken);
+
+        private async Task<ProjectionResult?> GetProjectionCoreAsync(LSPDocumentSnapshot documentSnapshot, Position position, bool rejectOnNewerParallelRequest, CancellationToken cancellationToken)
         {
             if (documentSnapshot is null)
             {
@@ -126,7 +132,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
             }
             else
             {
-                var synchronized = await _documentSynchronizer.TrySynchronizeVirtualDocumentAsync(documentSnapshot.Version, virtualDocument, cancellationToken).ConfigureAwait(false);
+                var synchronized = await _documentSynchronizer.TrySynchronizeVirtualDocumentAsync(documentSnapshot.Version, virtualDocument, rejectOnNewerParallelRequest, cancellationToken).ConfigureAwait(false);
                 if (!synchronized)
                 {
                     _logHubLogger.LogInformation("Could not synchronize.");

src/Razor/src/Microsoft.VisualStudio.LanguageServerClient.Razor/HtmlCSharp/LSPProjectionProvider.cs

@@ -13,5 +13,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
     internal abstract class LSPProjectionProvider
     {
         public abstract Task<ProjectionResult?> GetProjectionAsync(LSPDocumentSnapshot documentSnapshot, Position position, CancellationToken cancellationToken);
+
+        public abstract Task<ProjectionResult?> GetProjectionForCompletionAsync(LSPDocumentSnapshot documentSnapshot, Position position, CancellationToken cancellationToken);
     }
 }

src/Razor/test/Microsoft.VisualStudio.LanguageServer.ContainedLanguage.Test/DefaultLSPDocumentSynchronizerTest.cs

@@ -175,6 +175,66 @@ namespace Microsoft.VisualStudio.LanguageServer.ContainedLanguage
             Assert.True(result2);
         }
 
+        [Fact]
+        public async Task TrySynchronizeVirtualDocumentAsync_NewerVersionRequested_CancelsActiveRequest()
+        {
+            // Arrange
+            var (lspDocument, virtualDocument) = CreateDocuments(lspDocumentVersion: 124, virtualDocumentSyncVersion: 123);
+            var fileUriProvider = CreateUriProviderFor(VirtualDocumentTextBuffer, virtualDocument.Uri);
+            var synchronizer = new DefaultLSPDocumentSynchronizer(fileUriProvider)
+            {
+                _synchronizationTimeout = TimeSpan.FromMilliseconds(500)
+            };
+            NotifyLSPDocumentAdded(lspDocument, synchronizer);
+
+            // Act
+
+            // Start synchronization, this will hang until we notify the buffer versions been updated because the above virtual document expects host doc version 123 but the host doc is 124
+            var synchronizeTask1 = synchronizer.TrySynchronizeVirtualDocumentAsync(lspDocument.Version, virtualDocument, CancellationToken.None);
+            var (newLSPDocument, newVirtualDocument) = CreateDocuments(lspDocumentVersion: 125, virtualDocumentSyncVersion: 124);
+            var synchronizeTask2 = synchronizer.TrySynchronizeVirtualDocumentAsync(newLSPDocument.Version, newVirtualDocument, CancellationToken.None);
+
+            NotifyBufferVersionUpdated(VirtualDocumentTextBuffer, lspDocument.Version);
+            NotifyBufferVersionUpdated(VirtualDocumentTextBuffer, newLSPDocument.Version);
+
+            var result1 = await synchronizeTask1.ConfigureAwait(false);
+            var result2 = await synchronizeTask2.ConfigureAwait(false);
+
+            // Assert
+            Assert.False(result1);
+            Assert.True(result2);
+        }
+
+        [Fact]
+        public async Task TrySynchronizeVirtualDocumentAsync_RejectOnNewerParallelRequest_NewerVersionRequested_CancelsActiveRequest()
+        {
+            // Arrange
+            var (lspDocument, virtualDocument) = CreateDocuments(lspDocumentVersion: 124, virtualDocumentSyncVersion: 123);
+            var fileUriProvider = CreateUriProviderFor(VirtualDocumentTextBuffer, virtualDocument.Uri);
+            var synchronizer = new DefaultLSPDocumentSynchronizer(fileUriProvider)
+            {
+                _synchronizationTimeout = TimeSpan.FromMilliseconds(500)
+            };
+            NotifyLSPDocumentAdded(lspDocument, synchronizer);
+
+            // Act
+
+            // Start synchronization, this will hang until we notify the buffer versions been updated because the above virtual document expects host doc version 123 but the host doc is 124
+            var synchronizeTask1 = synchronizer.TrySynchronizeVirtualDocumentAsync(lspDocument.Version, virtualDocument, rejectOnNewerParallelRequest: false, CancellationToken.None);
+            var (newLSPDocument, newVirtualDocument) = CreateDocuments(lspDocumentVersion: 125, virtualDocumentSyncVersion: 124);
+            var synchronizeTask2 = synchronizer.TrySynchronizeVirtualDocumentAsync(newLSPDocument.Version, newVirtualDocument, CancellationToken.None);
+
+            NotifyBufferVersionUpdated(VirtualDocumentTextBuffer, lspDocument.Version);
+            NotifyBufferVersionUpdated(VirtualDocumentTextBuffer, newLSPDocument.Version);
+
+            var result1 = await synchronizeTask1.ConfigureAwait(false);
+            var result2 = await synchronizeTask2.ConfigureAwait(false);
+
+            // Assert
+            Assert.True(result1);
+            Assert.True(result2);
+        }
+
         [Fact]
         public async Task TrySynchronizeVirtualDocumentAsync_DocumentRemoved_CancelsActiveRequests()
         {

src/Razor/test/Microsoft.VisualStudio.LanguageServerClient.Razor.Test/Debugging/DefaultRazorBreakpointResolverTest.cs

@@ -357,6 +357,9 @@ $@"public class SomeRazorFile
 
                 return Task.FromResult(projectionResult);
             }
+
+            public override Task<ProjectionResult> GetProjectionForCompletionAsync(LSPDocumentSnapshot documentSnapshot, Position position, CancellationToken cancellationToken)
+                => GetProjectionAsync(documentSnapshot, position, cancellationToken);
         }
     }
 }

src/Razor/test/Microsoft.VisualStudio.LanguageServerClient.Razor.Test/Debugging/DefaultRazorProximityExpressionResolverTest.cs

@@ -262,6 +262,9 @@ $@"public class SomeRazorFile
 
                 return Task.FromResult(projectionResult);
             }
+
+            public override Task<ProjectionResult> GetProjectionForCompletionAsync(LSPDocumentSnapshot documentSnapshot, Position position, CancellationToken cancellationToken)
+                => GetProjectionAsync(documentSnapshot, position, cancellationToken);
         }
     }
 }

src/Razor/test/Microsoft.VisualStudio.LanguageServerClient.Razor.Test/HtmlCSharp/CompletionHandlerTest.cs

@@ -86,7 +86,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
             documentManager.AddDocument(Uri, new TestLSPDocumentSnapshot(new Uri("C:/path/file.razor"), 0, CSharpVirtualDocumentSnapshot));
             var requestInvoker = Mock.Of<LSPRequestInvoker>(MockBehavior.Strict);
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict).Object;
-            Mock.Get(projectionProvider).Setup(projectionProvider => projectionProvider.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), CancellationToken.None))
+            Mock.Get(projectionProvider).Setup(projectionProvider => projectionProvider.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), CancellationToken.None))
                 .Returns(Task.FromResult<ProjectionResult>(null));
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker, documentManager, projectionProvider, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
             var completionRequest = new CompletionParams()
@@ -137,7 +137,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.Html,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -175,7 +175,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.Html,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -221,7 +221,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -272,7 +272,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -352,7 +352,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -409,7 +409,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -468,7 +468,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -548,7 +548,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, navigatorSelector, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -624,7 +624,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, navigatorSelector, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -680,7 +680,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.Html,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -712,7 +712,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -755,7 +755,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -802,7 +802,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -856,7 +856,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -940,7 +940,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, navigatorSelector, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -991,7 +991,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.Html,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -1101,8 +1101,8 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 HostDocumentVersion = 1,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), new Position(1, 6), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), completionRequest.Position, It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), new Position(1, 6), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), completionRequest.Position, It.IsAny<CancellationToken>())).Returns(Task.FromResult(projectionResult));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -1202,7 +1202,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 HostDocumentVersion = 1,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -1282,7 +1282,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.Html,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -1324,7 +1324,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 LanguageKind = RazorLanguageKind.CSharp,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -1387,7 +1387,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 HostDocumentVersion = null,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 
@@ -1452,7 +1452,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
                 HostDocumentVersion = 1,
             };
             var projectionProvider = new Mock<LSPProjectionProvider>(MockBehavior.Strict);
-            projectionProvider.Setup(p => p.GetProjectionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
+            projectionProvider.Setup(p => p.GetProjectionForCompletionAsync(It.IsAny<LSPDocumentSnapshot>(), It.IsAny<Position>(), It.IsAny<CancellationToken>())).Returns(Task.FromResult(previousCharacterProjection));
 
             var completionHandler = new CompletionHandler(JoinableTaskContext, requestInvoker.Object, documentManager, projectionProvider.Object, TextStructureNavigatorSelectorService, CompletionRequestContextCache, FormattingOptionsProvider, LoggerProvider);
 

src/Razor/test/Microsoft.VisualStudio.LanguageServerClient.Razor.Test/HtmlCSharp/DefaultLSPProjectionProviderTest.cs

@@ -88,7 +88,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
 
             var documentSynchronizer = new Mock<LSPDocumentSynchronizer>(MockBehavior.Strict);
             documentSynchronizer
-                .Setup(d => d.TrySynchronizeVirtualDocumentAsync(DocumentSnapshot.Version, HtmlVirtualDocumentSnapshot, It.IsAny<CancellationToken>()))
+                .Setup(d => d.TrySynchronizeVirtualDocumentAsync(DocumentSnapshot.Version, HtmlVirtualDocumentSnapshot, true, It.IsAny<CancellationToken>()))
                 .Returns(Task.FromResult(true));
 
             var projectionProvider = new DefaultLSPProjectionProvider(requestInvoker.Object, documentSynchronizer.Object, Mock.Of<RazorLogger>(MockBehavior.Strict), LoggerProvider);
@@ -127,7 +127,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
 
             var documentSynchronizer = new Mock<LSPDocumentSynchronizer>(MockBehavior.Strict);
             documentSynchronizer
-                .Setup(d => d.TrySynchronizeVirtualDocumentAsync(DocumentSnapshot.Version, CSharpVirtualDocumentSnapshot, It.IsAny<CancellationToken>()))
+                .Setup(d => d.TrySynchronizeVirtualDocumentAsync(DocumentSnapshot.Version, CSharpVirtualDocumentSnapshot, true, It.IsAny<CancellationToken>()))
                 .Returns(Task.FromResult(true));
 
             var projectionProvider = new DefaultLSPProjectionProvider(requestInvoker.Object, documentSynchronizer.Object, Mock.Of<RazorLogger>(MockBehavior.Strict), LoggerProvider);
@@ -166,7 +166,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
 
             var documentSynchronizer = new Mock<LSPDocumentSynchronizer>(MockBehavior.Strict);
             documentSynchronizer
-                .Setup(d => d.TrySynchronizeVirtualDocumentAsync(DocumentSnapshot.Version, HtmlVirtualDocumentSnapshot, It.IsAny<CancellationToken>()))
+                .Setup(d => d.TrySynchronizeVirtualDocumentAsync(DocumentSnapshot.Version, HtmlVirtualDocumentSnapshot, true, It.IsAny<CancellationToken>()))
                 .Returns(Task.FromResult(true));
 
             var logger = new Mock<RazorLogger>(MockBehavior.Strict);
@@ -183,6 +183,45 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
             Assert.Equal(expectedPosition, result.Position);
         }
 
+        [Fact]
+        public async Task GetProjectionForCompletionAsync_CSharpProjection_ReturnsProjection()
+        {
+            // Arrange
+            var expectedPosition = new Position(0, 0);
+            var response = new RazorLanguageQueryResponse()
+            {
+                Kind = RazorLanguageKind.CSharp,
+                HostDocumentVersion = 1,
+                Position = new Position(expectedPosition.Line, expectedPosition.Character)
+            };
+            var requestInvoker = new Mock<LSPRequestInvoker>(MockBehavior.Strict);
+            requestInvoker
+                .Setup(r => r.ReinvokeRequestOnServerAsync<RazorLanguageQueryParams, RazorLanguageQueryResponse>(
+                    It.IsAny<ITextBuffer>(),
+                    It.IsAny<string>(),
+                    RazorLSPConstants.RazorLanguageServerName,
+                    It.IsAny<Func<JToken, bool>>(),
+                    It.IsAny<RazorLanguageQueryParams>(),
+                    It.IsAny<CancellationToken>()))
+                .Returns(Task.FromResult(new ReinvocationResponse<RazorLanguageQueryResponse>("LanguageClient", response)));
+
+            var documentSynchronizer = new Mock<LSPDocumentSynchronizer>(MockBehavior.Strict);
+            documentSynchronizer
+                .Setup(d => d.TrySynchronizeVirtualDocumentAsync(DocumentSnapshot.Version, CSharpVirtualDocumentSnapshot, false, It.IsAny<CancellationToken>()))
+                .Returns(Task.FromResult(true));
+
+            var projectionProvider = new DefaultLSPProjectionProvider(requestInvoker.Object, documentSynchronizer.Object, Mock.Of<RazorLogger>(MockBehavior.Strict), LoggerProvider);
+
+            // Act
+            var result = await projectionProvider.GetProjectionForCompletionAsync(DocumentSnapshot, new Position(), CancellationToken.None).ConfigureAwait(false);
+
+            // Assert
+            Assert.NotNull(result);
+            Assert.Equal(CSharpVirtualDocumentSnapshot.Uri, result.Uri);
+            Assert.Equal(RazorLanguageKind.CSharp, result.LanguageKind);
+            Assert.Equal(expectedPosition, result.Position);
+        }
+
         [Fact]
         public async Task GetProjectionAsync_SynchronizationFails_ReturnsNull()
         {
@@ -207,7 +246,7 @@ namespace Microsoft.VisualStudio.LanguageServerClient.Razor.HtmlCSharp
 
             var documentSynchronizer = new Mock<LSPDocumentSynchronizer>(MockBehavior.Strict);
             documentSynchronizer
-                .Setup(d => d.TrySynchronizeVirtualDocumentAsync(DocumentSnapshot.Version, CSharpVirtualDocumentSnapshot, It.IsAny<CancellationToken>()))
+                .Setup(d => d.TrySynchronizeVirtualDocumentAsync(DocumentSnapshot.Version, CSharpVirtualDocumentSnapshot, true, It.IsAny<CancellationToken>()))
                 .Returns(Task.FromResult(false));
 
             var projectionProvider = new DefaultLSPProjectionProvider(requestInvoker.Object, documentSynchronizer.Object, Mock.Of<RazorLogger>(MockBehavior.Strict), LoggerProvider);