From 0474a35b2ed0428c174fb7fe92f147360b485a45 Mon Sep 17 00:00:00 2001
From: David Wengier <david.wengier@microsoft.com>
Date: Thu, 2 Jan 2025 11:49:33 +1100
Subject: [PATCH] Fix completion in an empty document

---
 .../IDocumentMappingServiceExtensions.cs      | 12 +++++
 .../CohostDocumentCompletionEndpointTest.cs   | 53 +++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/DocumentMapping/IDocumentMappingServiceExtensions.cs b/src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/DocumentMapping/IDocumentMappingServiceExtensions.cs
index ec358cd5540..7293971c6bd 100644
--- a/src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/DocumentMapping/IDocumentMappingServiceExtensions.cs
+++ b/src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/DocumentMapping/IDocumentMappingServiceExtensions.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT license. See License.txt in the project root for license information.
 
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
@@ -47,6 +48,17 @@ public static DocumentPositionInfo GetPositionInfo(
         int hostDocumentIndex)
     {
         var sourceText = codeDocument.Source.Text;
+
+        if (sourceText.Length == 0)
+        {
+            Debug.Assert(hostDocumentIndex == 0);
+
+            // Special case for empty documents, to just force Html. When there is no content, then there are no source mappings,
+            // so the map call below fails, and we would default to Razor. This is fine for most cases, but empty documents are a
+            // special case where Html provides much better results when users first start typing.
+            return new DocumentPositionInfo(RazorLanguageKind.Html, new Position(0, 0), hostDocumentIndex);
+        }
+
         var position = sourceText.GetPosition(hostDocumentIndex);
 
         var languageKind = codeDocument.GetLanguageKind(hostDocumentIndex, rightAssociative: false);
diff --git a/src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Cohost/CohostDocumentCompletionEndpointTest.cs b/src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Cohost/CohostDocumentCompletionEndpointTest.cs
index 685ce73e866..a9413b4ce9d 100644
--- a/src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Cohost/CohostDocumentCompletionEndpointTest.cs
+++ b/src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Cohost/CohostDocumentCompletionEndpointTest.cs
@@ -313,6 +313,59 @@ The end.
              snippetLabels: ["snippet1", "snippet2"]);
     }
 
+    [Fact]
+    public async Task HtmlSnippetsCompletion_EmptyDocument()
+    {
+        await VerifyCompletionListAsync(
+            input: """
+                $$
+                """,
+             completionContext: new RoslynVSInternalCompletionContext()
+             {
+                 InvokeKind = RoslynVSInternalCompletionInvokeKind.Explicit,
+                 TriggerCharacter = null,
+                 TriggerKind = RoslynCompletionTriggerKind.Invoked
+             },
+             expectedItemLabels: ["snippet1", "snippet2"],
+             snippetLabels: ["snippet1", "snippet2"]);
+    }
+
+    [Fact]
+    public async Task HtmlSnippetsCompletion_WhitespaceOnlyDocument1()
+    {
+        await VerifyCompletionListAsync(
+            input: """
+
+                $$
+                """,
+             completionContext: new RoslynVSInternalCompletionContext()
+             {
+                 InvokeKind = RoslynVSInternalCompletionInvokeKind.Explicit,
+                 TriggerCharacter = null,
+                 TriggerKind = RoslynCompletionTriggerKind.Invoked
+             },
+             expectedItemLabels: ["snippet1", "snippet2"],
+             snippetLabels: ["snippet1", "snippet2"]);
+    }
+
+    [Fact]
+    public async Task HtmlSnippetsCompletion_WhitespaceOnlyDocument2()
+    {
+        await VerifyCompletionListAsync(
+            input: """
+                $$
+
+                """,
+             completionContext: new RoslynVSInternalCompletionContext()
+             {
+                 InvokeKind = RoslynVSInternalCompletionInvokeKind.Explicit,
+                 TriggerCharacter = null,
+                 TriggerKind = RoslynCompletionTriggerKind.Invoked
+             },
+             expectedItemLabels: ["snippet1", "snippet2"],
+             snippetLabels: ["snippet1", "snippet2"]);
+    }
+
     [Fact]
     public async Task HtmlSnippetsCompletion_NotInStartTag()
     {