Refactor Actions.tsx output rendering and terminal-state handling

[jlewi]

Summary

Actions.tsx currently mixes notebook mutation concerns, output presentation policy, terminal-console rendering, and legacy renderer registration. The recent getAllRenderersRef bug exposed one concrete failure mode, but it also points to a broader design problem in how terminal-oriented outputs are represented and rendered.

Problems

• Actions.updateCellLocal(...) still calls renderer hooks as part of local notebook mutation.
• StatefulRunmeTerminal is used as both a rendering hint and a control marker in notebook outputs.
• Terminal rendering decisions are split across multiple signals:
  • output MIME markers
  • active stream presence
  • runner / execution state
• Actions.tsx currently owns both output rendering policy and UI implementation details for CellConsole.
• The renderer registry appears to be mostly legacy, but parts of the output path still depend on it indirectly.

Why This Matters

The notebook architecture is model/view:

1. mutate NotebookData
2. emit model changes
3. let React subscribers re-render from model state

The current output path makes it hard to preserve that boundary cleanly. It also makes it unclear whether terminal presentation should be:

• inferred directly from model + run state
• represented explicitly in notebook state
• or derived from a renderer abstraction

That ambiguity is what led to renderer callbacks participating in mutation semantics.

Refactor Goals

• Remove any renderer callback from notebook mutation paths.
• Clarify the single source of truth for terminal-style rendering.
• Decide whether StatefulRunmeTerminal remains a real model concept or should be removed.
• Split Actions.tsx responsibilities so output policy is easier to reason about and test.
• Reduce special-case coupling between notebook outputs, stream state, and CellConsole.

Candidate Work

• Audit all remaining uses of StatefulRunmeTerminal.
• Decide whether terminal rendering should be inferred from run metadata / stream state instead of seeded outputs.
• Extract output rendering policy from Actions.tsx into smaller helpers or components.
• Remove legacy renderer-registration paths if no longer needed.
• Add tests that cover:
  • live terminal rendering
  • rerender after stream attach/detach
  • persisted output replay
  • stdout/stderr display rules for terminal vs non-terminal cells

Relationship To The Immediate Bug Fix

The immediate getAllRenderersRef fix should remove renderer callbacks from mutation paths first. That change does not need to block on a larger Actions.tsx refactor, but the refactor should be tracked separately because the current output architecture still carries avoidable complexity.