Allow Razor completion items on deletion (#11991)

* We had very old code that explicitely prohibited Razor items from being returned on deletion. I tested it and it seems to work well now, so I am removing the code blocking us from returning Razor completion items on deletion.

* Fixing up unit test to account for changed behavior

Author: alexgav

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Completion/RazorCompletionListProvider.cs

@@ -263,12 +263,6 @@ internal static bool IsApplicableTriggerContext(CompletionContext context)
             return true;
         }
 
-        if (vsCompletionContext.InvokeKind == VSInternalCompletionInvokeKind.Deletion)
-        {
-            // We do not support providing completions on delete.
-            return false;
-        }
-
         return true;
     }
 }

src/Razor/test/Microsoft.CodeAnalysis.Razor.Workspaces.Test/Completion/RazorCompletionListProviderTest.cs

@@ -62,7 +62,7 @@ private static IRazorCompletionItemProvider[] GetCompletionProviders()
         ];
 
     [Fact]
-    public void IsApplicableTriggerContext_Deletion_ReturnsFalse()
+    public void IsApplicableTriggerContext_Deletion_ReturnsTrue()
     {
         // Arrange
         var completionContext = new VSInternalCompletionContext()
@@ -74,7 +74,7 @@ public void IsApplicableTriggerContext_Deletion_ReturnsFalse()
         var result = RazorCompletionListProvider.IsApplicableTriggerContext(completionContext);
 
         // Assert
-        Assert.False(result);
+        Assert.True(result);
     }
 
     [Fact]

src/Razor/test/Microsoft.VisualStudio.Razor.IntegrationTests/CompletionIntegrationTests.cs

@@ -562,6 +562,31 @@ public void IncrementCount()
         await TestServices.Editor.ValidateNoDiscoColorsAsync(HangMitigatingCancellationToken);
     }
 
+    [IdeFact]
+    [WorkItem("https://github.com/dotnet/razor/issues/11565")]
+    public async Task TagHelpers_Present_OnBackspace()
+    {
+        await TestServices.SolutionExplorer.AddFileAsync(
+            RazorProjectConstants.BlazorProjectName,
+            "Test.razor",
+            """
+            <PageTitle>Test</PageTitle>
+            """,
+            open: true,
+            ControlledHangMitigatingCancellationToken);
+
+        await TestServices.Editor.WaitForComponentClassificationAsync(ControlledHangMitigatingCancellationToken);
+
+        await TestServices.Editor.PlaceCaretAsync("<PageTitle", charsOffset: 1, ControlledHangMitigatingCancellationToken);
+        TestServices.Input.Send("{BACKSPACE}");
+
+        var completionSession = await TestServices.Editor.WaitForCompletionSessionAsync(s_snippetTimeout, HangMitigatingCancellationToken);
+        var items = completionSession?.GetComputedItems(HangMitigatingCancellationToken);
+
+        Assert.NotNull(items);
+        Assert.Contains("PageTitle", items.Items.Select(i => i.DisplayText));
+    }
+
     private async Task VerifyTypeAndCommitCompletionAsync(string input, string output, string search, string[] stringsToType, char? commitChar = null, string? expectedSelectedItemLabel = null)
     {
         await TestServices.SolutionExplorer.AddFileAsync(