Don't compute the information for the code definition window if not open

If the symbol your caret is over is from metadata, producing the
output might be really expensive, so let's not do it in that case. The
window clears anything when it's opened, so the user won't know it was
never doing anything when closed.

src/EditorFeatures/Core/Implementation/CodeDefinitionWindow/DefinitionContextTracker.cs

@@ -124,6 +124,14 @@ private async Task UpdateForCaretPositionAsync(SnapshotPoint pointInRoslynSnapsh
             {
                 await _asyncListener.Delay(TimeSpan.FromMilliseconds(250), cancellationToken).ConfigureAwait(false);
 
+                // If it's not open, don't do anything, since if we are going to show locations in metadata that might
+                // be expensive. This doesn't cause a functional issue, since opening the window clears whatever was previously there
+                // so the user won't notice we weren't doing anything when it was open.
+                if (!await _codeDefinitionWindowService.IsWindowOpenAsync(cancellationToken).ConfigureAwait(false))
+                {
+                    return;
+                }
+
                 var document = pointInRoslynSnapshot.Snapshot.GetOpenDocumentInCurrentContextWithChanges();
                 if (document == null)
                 {

src/EditorFeatures/Core/Implementation/CodeDefinitionWindow/ICodeDefinitionWindowService.cs

@@ -10,6 +10,7 @@ namespace Microsoft.CodeAnalysis.Editor.Implementation.CodeDefinitionWindow
 {
     internal interface ICodeDefinitionWindowService
     {
+        Task<bool> IsWindowOpenAsync(CancellationToken cancellationToken);
         Task SetContextAsync(ImmutableArray<CodeDefinitionWindowLocation> locations, CancellationToken cancellationToken);
     }
 }

src/EditorFeatures/Test2/CodeDefinitionWindow/AbstractCodeDefinitionWindowTests.vb

@@ -30,6 +30,10 @@ Namespace Microsoft.CodeAnalysis.Editor.CodeDefinitionWindow.UnitTests
             Public Sub New()
             End Sub
 
+            Public Function IsWindowOpenAsync(cancellationToken As CancellationToken) As Task(Of Boolean) Implements ICodeDefinitionWindowService.IsWindowOpenAsync
+                Throw New NotImplementedException()
+            End Function
+
             Public Function SetContextAsync(locations As ImmutableArray(Of CodeDefinitionWindowLocation), cancellationToken As CancellationToken) As Task Implements ICodeDefinitionWindowService.SetContextAsync
                 Throw New NotImplementedException()
             End Function

src/VisualStudio/Core/Def/Implementation/CodeDefinitionWindow/VisualStudioCodeDefinitionWindowService.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Immutable;
 using System.Composition;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Runtime.InteropServices;
 using System.Threading;
@@ -23,6 +24,9 @@ internal sealed class VisualStudioCodeDefinitionWindowService : ICodeDefinitionW
         private readonly IAsyncServiceProvider _asyncServiceProvider;
         private readonly IThreadingContext _threadingContext;
 
+        [DisallowNull]
+        private IVsCodeDefView? _lazyCodeDefView;
+
         [ImportingConstructor]
         [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
         public VisualStudioCodeDefinitionWindowService(SVsServiceProvider asyncServiceProvider, IThreadingContext threadingContext)
@@ -31,6 +35,29 @@ public VisualStudioCodeDefinitionWindowService(SVsServiceProvider asyncServicePr
             _threadingContext = threadingContext;
         }
 
+        private async Task<IVsCodeDefView> GetVsCodeDefViewAsync()
+        {
+            if (_lazyCodeDefView is not null)
+            {
+                return _lazyCodeDefView;
+            }
+
+            _lazyCodeDefView = await _asyncServiceProvider.GetServiceAsync<SVsCodeDefView, IVsCodeDefView>().ConfigureAwait(false);
+
+            return _lazyCodeDefView;
+        }
+
+        public async Task<bool> IsWindowOpenAsync(CancellationToken cancellationToken)
+        {
+            var vsCodeDefView = await GetVsCodeDefViewAsync().ConfigureAwait(true);
+
+            // Switch to the UI thread before using the IVsCodeDefView service
+            await _threadingContext.JoinableTaskFactory.SwitchToMainThreadAsync(cancellationToken);
+
+            // IsVisible returns S_FALSE if it's not visible
+            return vsCodeDefView.IsVisible() == VSConstants.S_OK;
+        }
+
         public async Task SetContextAsync(ImmutableArray<CodeDefinitionWindowLocation> locations, CancellationToken cancellationToken)
         {
             // If the new context has no location, then just don't update, instead of showing the