• VS Code Version: 1.56.0-insider (user setup)
• OS Version: Windows_NT x64 10.0.19042
• Electron: 12.0.4
• Chrome: 89.0.4389.114
• Node.js: 14.16.0
• V8: 8.9.255.24-electron.0

Steps to Reproduce:

1. Open a markdown file, and click the "Open preview to the side" button
2. The preview window is blank, and an error notification pops up in the bottom right:

Error loading webview: Error: Could not register service workers: TypeError: Failed to register a ServiceWorker for scope ('vscode-webview://36f8a74c-37c6-4d60-9d4c-87414f69ebcc/') with script ('vscode-webview://36f8a74c-37c6-4d60-9d4c-87414f69ebcc/service-worker.js?platform=electron&id=36f8a74c-37c6-4d60-9d4c-87414f69ebcc&vscode-resource-origin=https%3A%2F%2F36f8a74c-37c6-4d60-9d4c-87414f69ebcc.vscode-webview-test.com'): ServiceWorker cannot be started.

The same thing happens for other WebViews, like opening an extension from the Extensions sidebar, or using the "Issue Reporter".

Does this issue occur when all extensions are disabled?: Yes