update

src/kernels/kernel.ts

@@ -196,6 +196,10 @@ abstract class BaseKernel implements IBaseKernel {
     public get restarting() {
         return this._restartPromise || Promise.resolve();
     }
+    private _postInitializingDeferred = createDeferred<void>();
+    public get postInitializing() {
+        return this._postInitializingDeferred.promise;
+    }
     constructor(
         public readonly id: string,
         public readonly uri: Uri,
@@ -258,9 +262,14 @@ abstract class BaseKernel implements IBaseKernel {
             // If we started and the UI is no longer disabled (ie., a user executed a cell)
             // then we can signal that the kernel was created and can be used by third-party extensions.
             // We also only want to fire off a single event here.
-            if (!options?.disableUI && !this._postInitializedOnStart) {
+            if (!this.startupUI.disableUI && !this._postInitializedOnStart) {
                 this._onPostInitialized.fire();
                 this._postInitializedOnStart = true;
+                // Inject a small delay to allow external event handlers a brief window
+                // to queue code executions.
+                setTimeout(() => {
+                    this._postInitializingDeferred.resolve();
+                }, 50);
             }
             return result;
         });

src/kernels/kernelExecution.ts

@@ -166,8 +166,13 @@ export class NotebookKernelExecution implements INotebookKernelExecution {
         const sessionPromise = this.kernel.restarting.then(() => this.kernel.start(new DisplayOptions(false)));
 
         traceCellMessage(cell, `NotebookKernelExecution.executeCell (3), ${getDisplayPath(cell.notebook.uri)}`);
+
+        // Wait for the kernel to complete post initialization before queueing the cell in case
+        // we need to allow extensions to run code before the initial user-triggered execution
+        await this.kernel.postInitializing;
         const executionQueue = this.getOrCreateCellExecutionQueue(cell.notebook, sessionPromise);
         executionQueue.queueCell(cell, codeOverride);
+
         let success = true;
         try {
             traceCellMessage(cell, `NotebookKernelExecution.executeCell (4), ${getDisplayPath(cell.notebook.uri)}`);

src/kernels/types.ts

@@ -357,6 +357,7 @@ export interface IBaseKernel extends IAsyncDisposable {
     readonly onRestarted: Event<void>;
     readonly onPostInitialized: Event<void>;
     readonly restarting: Promise<void>;
+    readonly postInitializing: Promise<void>;
     readonly status: KernelMessage.Status;
     readonly disposed: boolean;
     readonly disposing: boolean;