"The active document is not part of the open workspace. Not all language features will be available." inside dev container

Issue originally posted to https://github.com/microsoft/vscode-dotnettools/issues/3331, but after some further testing it seems specific to dev containers.

• VSCode Version

Version: 1.133.0-insider (user setup)
Commit: 8eaafbf4220a3cb099d7bd99c8d04dca7062236f
Date: 2026-08-07T16:53:13Z
Electron: 42.8.0
ElectronBuildId: 14845705
Chromium: 148.0.7778.280
Node.js: 24.18.0
V8: 14.8.178.38-electron.0
@github/copilot: 1.0.79-2
@github/copilot-sdk: 1.0.9-preview.3
OS: 8eaafbf4220a3cb099d7bd99c8d04dca7062236f

• Local OS Version: Microsoft Windows Version 25H2 (OS Build 26200.8973)
• Remote OS Version: Docker Desktop on same Windows host using WSL2 backend
• Remote Extension/Connection Type: Dev Containers
• Logs:
  devcontainer-log.txt

Steps to Reproduce:

1. Clone https://github.com/cyungmann/devcontainer-issue-repro
2. Open dev container using VS Code
3. Add a new *.cs file under src/CommandLine/ (e.g. src/CommandLine/Qux.cs)
4. Observe warning toast message The active document is not part of the open workspace. Not all language features will be available.
5. Attempt to reference a class definition in the new file from src/CommandLine/Program.cs and observe that the identifier for the class is not recognized:

Does this issue occur when you try this locally?: No
Does this issue occur when you try this locally and all extensions are disabled?: N/A