Positron nb focus on open

[nstrayer]

Addresses #9707.

When opening a Positron notebook, the editor was not properly focusing a cell, requiring users to click a cell before keyboard shortcuts like j and k for navigation would work. This PR fixes the issue by replacing the no selection state with a no-cells state in the selection state machine and adding an invariant to ensure at least one cell is always selected when the notebook contains cells.

I took this opportunity to refactor the selection state machine too as it had fallen out of good state-machine form after piecemeal edits for a while. Docs have been added at the top and we now verify transition correctness and throw errors in dev mode to hopefully catch bugs (It already caught one while making this PR.)

Release Notes

New Features

• N/A

Bug Fixes

• Fix Positron notebooks not focusing a cell on open, requiring click before keyboard navigation works (Positron Notebooks: Opening a notebook doesn't correctly set focus #9707)

QA Notes

@:positron-notebooks

The test suite includes comprehensive coverage of the fix:

1. Basic focus on open: Open a notebook file (e.g., via Cmd+P) and verify the first cell is automatically selected
2. Immediate keyboard navigation: Without clicking any cell, press J or K to verify navigation works immediately
3. Selection persistence: Switch between editors and verify cell selection is maintained when returning to the notebook

To manually test:

1. Open any notebook file using Cmd+P (or Ctrl+P on Windows/Linux)
2. Without clicking anywhere, press J or K
3. Expected: Cell selection moves down/up immediately
4. Previously: Nothing happened until clicking a cell

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:positron-notebooks

^{readme  valid tags}

[Copilot]

Pull Request Overview

This PR fixes an issue where Positron notebooks didn't automatically focus a cell when opened, requiring users to click before keyboard navigation would work. The fix replaces the NoSelection state with a NoCells state in the selection state machine and adds an invariant to ensure at least one cell is always selected when cells exist in the notebook.

Key changes:

• Refactored the selection state machine with comprehensive documentation and transition validation
• Added automatic cell selection when notebooks are opened
• Enhanced test coverage with new focus and selection behavior tests

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

File	Description
src/vs/workbench/contrib/positronNotebook/browser/selectionMachine.ts	Core fix: refactored state machine with NoCells state and invariant enforcement for automatic selection
test/e2e/tests/notebook/notebook-focus-and-selection.test.ts	Added comprehensive tests for initial focus behavior and keyboard navigation without clicks
test/e2e/infra/test-runner/test-tags.ts	Added POSITRON_NOTEBOOKS test tag for categorizing Positron-specific notebook tests
Multiple test files	Updated test descriptions to include the new POSITRON_NOTEBOOKS tag for proper test organization

[midleman]

Looks good from test perspective! As mentioned in a comment, I'm going to do a pass through of all the positron nb e2e test code and will likely make some infra optimizations. But... can you also tag this PR for @:win and re-run? Basically, anytime we add a new test we def want to make sure it passes there as well... because well... Windows. 😆

[seeM]

Code changes LGTM! A couple questions about the spec

[Copilot]

Pull Request Overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated no new comments.

[nstrayer]

Test failure is a data explorer test that seems flakey.

[seeM]

I like it!