Jupyter/Python extension hangs indefinitely when a notebook is restored on startup, but works fine when opened manually

Type: Bug

**Environment:** Windows, VS Code with a Miniconda-managed Python environment, Jupyter + Python extensions installed.

**Steps:**
1. Open VS Code with a folder containing a `.ipynb` notebook, with a conda environment available.
2. Open the notebook, select a conda kernel, and run a cell — it works correctly.
3. Close VS Code without closing the notebook tab.
4. Reopen VS Code. It automatically restores the previously opened notebook tab (default behavior).

**Actual result:**
The Python/Jupyter extension appears to load indefinitely. The kernel never connects, and the notebook stays unresponsive. Commands such as `Python: Clear Cache and Reload Window` returns: Command 'Python: Clear Cache and Reload Window' resulted in an error - command 'python.clearCacheAndReload' not found, which suggests the Python extension itself has not finished activating by the time the notebook editor tries to use it.

**Expected result:**
The notebook and its kernel should connect reliably on startup, the same way they do when opened manually (see workaround below).

**Workaround found:**
If I close the notebook tab, fully close VS Code, reopen VS Code (with no notebook restored), and then manually reopen the same notebook from the Explorer, the kernel connects immediately and the conda environment is detected without issues.

This points to a race condition between the notebook editor being restored on startup and the Jupyter/Python extensions finishing activation in the extension host.

VS Code version: Code 1.135.0 (08d4889f9ec4a1685d257b9b95de036c8e1ce1e5, 2026-08-25T14:26:52Z)
OS version: Windows_NT x64 10.0.19045
Modes:

System Info

|Item|Value|
|---|---|
|CPUs|AMD Ryzen 7 1700X Eight-Core Processor (16 x 3768)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x10de, DEVICE=0x1b80 [NVIDIA GeForce GTX 1080], DRIVER_VENDOR=NVIDIA, DRIVER_VERSION=32.0.15.8266 *ACTIVE*
GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.19041.7548
Machine model name:
Machine model version:
direct_rendering_display_compositor: disabled_off_ok
gpu_compositing: enabled
multiple_raster_threads: enabled_on
opengl: enabled_on
rasterization: enabled
raw_draw: disabled_off_ok
skia_graphite: disabled_off
trees_in_viz: disabled_off
video_decode: enabled
video_encode: enabled
webgl: enabled
webgpu: enabled
webnn: disabled_off|
|Load (avg)|undefined|
|Memory (System)|31.92GB (17.61GB free)|
|Process Argv|--crash-reporter-id e555bdfd-d4ae-478d-8fe1-289b24c7955e|
|Screen Reader|no|
|VM|0%|
Extensions (61)

Name|Identifier|Author|Version
---|---|---|---
CodeSnap|adpyke.codesnap|adpyke|1.3.4
Sublime Monokai|aryanahire.sublime-monokai|AryanAhire|0.2.9
Docker DX|docker.docker|docker|0.18.0
GitHub Repositories|github.remotehub|GitHub|0.64.0
GitHub Actions|github.vscode-github-actions|github|0.32.3
Rainbow CSV|mechatroner.rainbow-csv|mechatroner|3.24.1
Monokai Pro|monokai.theme-monokai-pro-vscode|monokai|2.0.14
Azure Developer CLI|ms-azuretools.azure-dev|ms-azuretools|0.10.0
GitHub Copilot for Azure|ms-azuretools.vscode-azure-github-copilot|ms-azuretools|1.0.231
Azure MCP Server|ms-azuretools.vscode-azure-mcp-server|ms-azuretools|2.0.46
Azure App Service|ms-azuretools.vscode-azureappservice|ms-azuretools|0.27.0
Azure Container Apps|ms-azuretools.vscode-azurecontainerapps|ms-azuretools|0.11.2
Azure Functions|ms-azuretools.vscode-azurefunctions|ms-azuretools|1.22.1
Azure Resources|ms-azuretools.vscode-azureresourcegroups|ms-azuretools|0.12.7
Azure Static Web Apps|ms-azuretools.vscode-azurestaticwebapps|ms-azuretools|0.13.3
Azure Storage|ms-azuretools.vscode-azurestorage|ms-azuretools|0.17.2
Azure Virtual Machines|ms-azuretools.vscode-azurevirtualmachines|ms-azuretools|0.6.11
Container Tools|ms-azuretools.vscode-containers|ms-azuretools|2.5.0
Azure Cosmos DB|ms-azuretools.vscode-cosmosdb|ms-azuretools|0.36.2
Docker|ms-azuretools.vscode-docker|ms-azuretools|2.0.0
C#|ms-dotnettools.csharp|ms-dotnettools|2.140.9
.NET Install Tool|ms-dotnettools.vscode-dotnet-runtime|ms-dotnettools|3.1.0
Kubernetes|ms-kubernetes-tools.vscode-kubernetes-tools|ms-kubernetes-tools|1.4.1
Python Debugger|ms-python.debugpy|ms-python|2026.6.0
Python|ms-python.python|ms-python|2026.4.0
Pylance|ms-python.vscode-pylance|ms-python|2026.3.1
Python Environments|ms-python.vscode-python-envs|ms-python|1.36.0
Data Wrangler|ms-toolsai.datawrangler|ms-toolsai|1.24.2
Jupyter|ms-toolsai.jupyter|ms-toolsai|2025.9.1
Jupyter Keymap|ms-toolsai.jupyter-keymap|ms-toolsai|1.1.2
Jupyter Notebook Renderers|ms-toolsai.jupyter-renderers|ms-toolsai|1.3.0
Azure Machine Learning|ms-toolsai.vscode-ai|ms-toolsai|1.4.2
Azure Machine Learning - Remote|ms-toolsai.vscode-ai-remote|ms-toolsai|1.4.9
Jupyter Cell Tags|ms-toolsai.vscode-jupyter-cell-tags|ms-toolsai|0.1.9
Jupyter PowerToys|ms-toolsai.vscode-jupyter-powertoys|ms-toolsai|0.1.1
Jupyter Slide Show|ms-toolsai.vscode-jupyter-slideshow|ms-toolsai|0.1.6
Dev Containers|ms-vscode-remote.remote-containers|ms-vscode-remote|0.466.0
Remote - SSH|ms-vscode-remote.remote-ssh|ms-vscode-remote|0.128.0
Remote - SSH: Editing Configuration Files|ms-vscode-remote.remote-ssh-edit|ms-vscode-remote|0.87.0
WSL|ms-vscode-remote.remote-wsl|ms-vscode-remote|0.104.3
Azure Repos|ms-vscode.azure-repos|ms-vscode|0.40.0
Azure CLI Tools|ms-vscode.azurecli|ms-vscode|0.6.0
Remote Explorer|ms-vscode.remote-explorer|ms-vscode|0.5.0
Remote Repositories|ms-vscode.remote-repositories|ms-vscode|0.42.0
Azure Tools|ms-vscode.vscode-node-azure-pack|ms-vscode|1.8.2
Foundry Toolkit for VS Code|ms-windows-ai-studio.windows-ai-studio|ms-windows-ai-studio|1.6.10
Language Support for Java(TM) by Red Hat|redhat.java|redhat|1.55.0
YAML|redhat.vscode-yaml|redhat|1.24.0
SynthWave '84|robbowen.synthwave-vscode|RobbOwen|0.1.20
Better Jinja|samuelcolvin.jinjahtml|samuelcolvin|0.20.0
FiraCode font - Professional Font for Developers|seyyedkhandon.firacode|SeyyedKhandon|2.2.2
Zip File Explorer|slevesque.vscode-zipexplorer|slevesque|0.3.1
vscode-pdf|tomoki1207.pdf|tomoki1207|1.2.2
GitHub Copilot modernization|vscjava.migrate-java-to-azure|vscjava|1.23.0
Gradle for Java|vscjava.vscode-gradle|vscjava|3.18.0
Debugger for Java|vscjava.vscode-java-debug|vscjava|0.59.0
Project Manager for Java|vscjava.vscode-java-dependency|vscjava|0.27.6
Extension Pack for Java|vscjava.vscode-java-pack|vscjava|0.31.1
Test Runner for Java|vscjava.vscode-java-test|vscjava|0.46.0
Maven for Java|vscjava.vscode-maven|vscjava|0.45.3
JavaScript (ES6) code snippets|xabikos.javascriptsnippets|xabikos|1.8.0

A/B Experiments

```
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jf4hg949:31526829
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session-mark-done:31558131
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signed-out-control:31574474
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permission_prompt_control:1332567
ccr_pr_nudge_adoption:1319471
vsc_wsm_t:1340632
eh853263:1355661
auto_mode_treatment:1359304

```