*** DO NOT MERGE: Revert "Remove most remaining uses of WorkspaceChanged off the UI thread (dotnet#78778)" (dotnet#79366)

This reverts commit 67cce50, reversing
changes made to aafd6eb.

Going to run a test insertion to see if this is the cause of regressions
flagged in
https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/650773

Author: dibarbet

src/EditorFeatures/Core/Shared/Tagging/EventSources/TaggerEventSources.DocumentActiveContextChangedEventSource.cs

@@ -13,8 +13,9 @@ private sealed class DocumentActiveContextChangedEventSource(ITextBuffer subject
     {
         private WorkspaceEventRegistration? _documentActiveContextChangedDisposer;
 
+        // Require main thread on the callback as RaiseChanged implementors may have main thread dependencies.
         protected override void ConnectToWorkspace(Workspace workspace)
-            => _documentActiveContextChangedDisposer = workspace.RegisterDocumentActiveContextChangedHandler(OnDocumentActiveContextChanged);
+            => _documentActiveContextChangedDisposer = workspace.RegisterDocumentActiveContextChangedHandler(OnDocumentActiveContextChanged, WorkspaceEventOptions.RequiresMainThreadOptions);
 
         protected override void DisconnectFromWorkspace(Workspace workspace)
             => _documentActiveContextChangedDisposer?.Dispose();

src/EditorFeatures/Core/Tagging/ITaggerEventSource.cs

@@ -41,7 +41,7 @@ internal interface ITaggerEventSource
 
     /// <summary>
     /// An event has happened on the thing the tagger is attached to.  The tagger should
-    /// recompute tags. May be raised on any thread.
+    /// recompute tags.
     /// </summary>
     event EventHandler<TaggerEventArgs> Changed;
 }

src/Features/Core/Portable/Diagnostics/CodeAnalysisDiagnosticAnalyzerService.cs

@@ -49,7 +49,9 @@ public CodeAnalysisDiagnosticAnalyzerService(
             _workspace = workspace;
             _diagnosticAnalyzerService = _workspace.Services.GetRequiredService<IDiagnosticAnalyzerService>();
 
-            _ = workspace.RegisterWorkspaceChangedHandler(OnWorkspaceChanged);
+            // Main thread as OnWorkspaceChanged's call to IDiagnosticAnalyzerService.RequestDiagnosticRefresh isn't clear on
+            // threading requirements
+            _ = workspace.RegisterWorkspaceChangedHandler(OnWorkspaceChanged, WorkspaceEventOptions.RequiresMainThreadOptions);
         }
 
         private void OnWorkspaceChanged(WorkspaceChangeEventArgs e)

src/Features/Core/Portable/Diagnostics/IDiagnosticAnalyzerService.cs

@@ -22,9 +22,6 @@ internal interface IDiagnosticAnalyzerService : IWorkspaceService
     /// <summary>
     /// Re-analyze all projects and documents.  This will cause an LSP diagnostic refresh request to be sent.
     /// </summary>
-    /// <remarks>
-    /// This implementation must be safe to call on any thread.
-    /// </remarks>
     void RequestDiagnosticRefresh();
 
     /// <summary>

src/LanguageServer/Protocol/Workspaces/LspWorkspaceRegistrationService.cs

@@ -39,7 +39,9 @@ public virtual void Register(Workspace? workspace)
             m["WorkspacePartialSemanticsEnabled"] = workspace.PartialSemanticsEnabled;
         }, workspace));
 
-        var workspaceChangedDisposer = workspace.RegisterWorkspaceChangedHandler(OnLspWorkspaceChanged);
+        // Forward workspace change events for all registered LSP workspaces. Requires main thread as it
+        // fires LspSolutionChanged which hasn't been guaranteed to be thread safe.
+        var workspaceChangedDisposer = workspace.RegisterWorkspaceChangedHandler(OnLspWorkspaceChanged, WorkspaceEventOptions.RequiresMainThreadOptions);
 
         lock (_gate)
         {
@@ -92,7 +94,9 @@ public void Dispose()
     }
 
     /// <summary>
-    /// Indicates whether the LSP solution has changed in a non-tracked document context. May be raised on any thread.
+    /// Indicates whether the LSP solution has changed in a non-tracked document context.
+    /// 
+    /// <b>IMPORTANT:</b> Implementations of this event handler should do as little synchronous work as possible since this will block.
     /// </summary>
     public EventHandler<WorkspaceChangeEventArgs>? LspSolutionChanged;
 }

src/VisualStudio/Core/Def/StackTraceExplorer/StackTraceExplorerViewModel.cs

@@ -50,6 +50,9 @@ public StackTraceExplorerViewModel(IThreadingContext threadingContext, Workspace
         _threadingContext = threadingContext;
         _workspace = workspace;
 
+        // Main thread dependency as Workspace_WorkspaceChanged modifies an ObservableCollection
+        _ = workspace.RegisterWorkspaceChangedHandler(Workspace_WorkspaceChanged, WorkspaceEventOptions.RequiresMainThreadOptions);
+
         _classificationTypeMap = classificationTypeMap;
         _formatMap = formatMap;
 
@@ -100,6 +103,15 @@ private void CallstackLines_CollectionChanged(object sender, System.Collections.
         NotifyPropertyChanged(nameof(IsInstructionTextVisible));
     }
 
+    private void Workspace_WorkspaceChanged(WorkspaceChangeEventArgs e)
+    {
+        if (e.Kind == WorkspaceChangeKind.SolutionChanged)
+        {
+            Selection = null;
+            Frames.Clear();
+        }
+    }
+
     private FrameViewModel GetViewModel(ParsedFrame frame)
         => frame switch
         {

src/VisualStudio/Core/Impl/SolutionExplorer/DiagnosticItem/CpsDiagnosticItemSource.cs

@@ -19,7 +19,6 @@ internal sealed partial class CpsDiagnosticItemSource : BaseDiagnosticAndGenerat
 {
     private readonly IVsHierarchyItem _item;
     private readonly string _projectDirectoryPath;
-    private readonly string? _analyzerFilePath;
 
     private WorkspaceEventRegistration? _workspaceChangedDisposer;
 
@@ -38,8 +37,6 @@ public CpsDiagnosticItemSource(
         _item = item;
         _projectDirectoryPath = Path.GetDirectoryName(projectPath);
 
-        _analyzerFilePath = CpsUtilities.ExtractAnalyzerFilePath(_projectDirectoryPath, _item.CanonicalName);
-
         this.AnalyzerReference = TryGetAnalyzerReference(Workspace.CurrentSolution);
         if (this.AnalyzerReference == null)
         {
@@ -50,7 +47,9 @@ public CpsDiagnosticItemSource(
             // then connect to it.
             if (workspace.CurrentSolution.ContainsProject(projectId))
             {
-                _workspaceChangedDisposer = Workspace.RegisterWorkspaceChangedHandler(OnWorkspaceChangedLookForAnalyzer);
+                // Main thread dependency as OnWorkspaceChangedLookForAnalyzer accesses the IVsHierarchy
+                // and fires the PropertyChanged event 
+                _workspaceChangedDisposer = Workspace.RegisterWorkspaceChangedHandler(OnWorkspaceChangedLookForAnalyzer, WorkspaceEventOptions.RequiresMainThreadOptions);
                 item.PropertyChanged += IVsHierarchyItem_PropertyChanged;
 
                 // Now that we've subscribed, check once more in case we missed the event
@@ -119,11 +118,14 @@ private void OnWorkspaceChangedLookForAnalyzer(WorkspaceChangeEventArgs e)
             return null;
         }
 
-        if (string.IsNullOrEmpty(_analyzerFilePath))
+        var canonicalName = _item.CanonicalName;
+        var analyzerFilePath = CpsUtilities.ExtractAnalyzerFilePath(_projectDirectoryPath, canonicalName);
+
+        if (string.IsNullOrEmpty(analyzerFilePath))
         {
             return null;
         }
 
-        return project.AnalyzerReferences.FirstOrDefault(r => string.Equals(r.FullPath, _analyzerFilePath, StringComparison.OrdinalIgnoreCase));
+        return project.AnalyzerReferences.FirstOrDefault(r => string.Equals(r.FullPath, analyzerFilePath, StringComparison.OrdinalIgnoreCase));
     }
 }