Properly compare prefix length

when triggering completion on backspace.

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

@@ -465,7 +465,7 @@ public FilterResult(CompletionItem completionItem, string filterText, bool match
                 if (filterReason == CompletionFilterReason.BackspaceOrDelete &&
                     trigger.Kind == CompletionTriggerKind.Deletion)
                 {
-                    return item.FilterText.GetCaseInsensitivePrefixLength(filterText) > 0;
+                    return item.FilterText.GetCaseInsensitivePrefixLength(filterText) >= Math.Min(item.FilterText.Length, filterText.Length);
                 }
 
                 // If the user hasn't typed anything, and this item was preselected, or was in the

src/EditorFeatures/Test2/IntelliSense/CSharpCompletionCommandHandlerTests.vb

@@ -2887,6 +2887,29 @@ class C
                 state.SendInvokeCompletionList()
                 Await state.AssertCompletionSession()
             End Using
+        End Function
+		
+		<WorkItem(14704, "https://github.com/dotnet/roslyn/issues/14704")>
+        <WpfFact, Trait(Traits.Feature, Traits.Features.Completion)>
+        Public Async Function BackspaceTriggerSubstringMatching() As Task
+            Using state = TestState.CreateCSharpTestState(
+                              <Document>
+using System;
+class Program
+{
+    static void Main(string[] args)
+    {
+        if (Environment$$
+    }
+}
+                              </Document>)
+
+                Dim key = New OptionKey(CompletionOptions.TriggerOnDeletion, LanguageNames.CSharp)
+                state.Workspace.Options = state.Workspace.Options.WithChangedOption(key, True)
+
+                state.SendBackspace()
+                Await state.AssertSelectedCompletionItem(displayText:="Environment", isHardSelected:=True)
+            End Using
         End Function
     End Class
-End Namespace
+End Namespace
\ No newline at end of file