Merge pull request #18786 from rchande/fix18785

Check for prefix match before hard selecting when backspacing

src/EditorFeatures/Core/Implementation/IntelliSense/Completion/Controller.Session_FilterModel.cs

@@ -373,8 +373,15 @@ private bool IsAfterDot(Model model, ITextSnapshot textSnapshot, Dictionary<Text
 
                 if (bestFilterResult != null)
                 {
+                    // Only hard select this result if it's a prefix match
+                    // We need to do this so that 
+                    // * deleting and retyping a dot in a member access does not change the 
+                    //   text that originally appeared before the dot
+                    // * deleting through a word from the end keeps that word selected
+                    // This also preserves the behavior the VB had through Dev12.
+                    var hardSelect = bestFilterResult.Value.CompletionItem.FilterText.StartsWith(model.FilterText, StringComparison.CurrentCultureIgnoreCase);
                     return model.WithSelectedItem(bestFilterResult.Value.CompletionItem)
-                                .WithHardSelection(true)
+                                .WithHardSelection(hardSelect)
                                 .WithIsUnique(matchCount == 1);
                 }
                 else

src/EditorFeatures/Test2/IntelliSense/VisualBasicCompletionCommandHandlerTests.vb

@@ -2873,6 +2873,27 @@ End Class
             End Using
         End Function
 
+        <WorkItem(18785, "https://github.com/dotnet/roslyn/issues/18785")>
+        <WpfFact, Trait(Traits.Feature, Traits.Features.Completion)>
+        Public Async Function BackspaceSoftSelectionIfNotPrefixMatch() As Task
+            Using state = TestState.CreateVisualBasicTestState(
+                            <Document><![CDATA[
+Class C
+    Sub Do()
+        Dim x = new System.Collections.Generic.List(Of String)()
+        x.$$Add("stuff")
+    End Sub
+End Class
+]]></Document>)
+
+                state.SendBackspace()
+                Await state.AssertSelectedCompletionItem("x", isSoftSelected:=True)
+                state.SendTypeChars(".")
+                Await state.WaitForAsynchronousOperationsAsync()
+                Assert.Contains("x.Add", state.GetLineTextFromCaretPosition())
+            End Using
+        End Function
+
         <ExportLanguageService(GetType(ISnippetInfoService), LanguageNames.VisualBasic), System.Composition.Shared>
 		Friend Class MockSnippetInfoService
 			Implements ISnippetInfoService