fix: make threads share their accumulated output

[akshayka]

This PR makes mo.Threads share their creating cell's accumulated output list. This improves the experience of imperatively writing output with threads in two ways:

1. Multiple threads can now write to the output, without overwriting each other's outputs
2. Thread outputs are no longer automatically cleared when the spawning cell completes

To accommodate this, this PR abstracts away the underlying list of imperative outputs, and instead introduces a thread-safe container that manages access to the outputs data structure.

This change also makes mo.status.progress_bar thread-safe.

Follow up to #8413

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Feb 21, 2026 8:30pm

[Copilot]

Pull request overview

This PR updates marimo’s imperative output plumbing so mo.Thread instances share the spawning cell’s accumulated output container, enabling safe multi-thread appends and preventing thread output from being wiped when the parent cell completes. It also adds thread-safety to mo.status.progress_bar updates.

Changes:

• Introduce CellOutputList, a thread-safe container for imperative cell outputs, and make it the default for ExecutionContext.output.
• Update mo.output.* internals, kernel output caching, and post-exec output broadcast logic to work with CellOutputList.
• Make progress bar updates thread-safe and add runtime + smoke tests covering threaded output scenarios.

Reviewed changes

Copilot reviewed 12 out of 15 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/_runtime/test_threads.py	Adds coverage ensuring threads share the parent cell’s output container.
tests/_runtime/test_cell_output_list.py	Adds unit tests for CellOutputList behavior.
marimo/_smoke_tests/threads/threads.py	Smoke test for stdout redirection with threading.Thread vs mo.Thread.
marimo/_smoke_tests/threads/state_threading.py	Smoke test exercising state updates from a background mo.Thread.
marimo/_smoke_tests/threads/progress_bar_threads.py	Smoke test for progress bar updates from multiple threads.
marimo/_smoke_tests/threads/multiple_appends.py	Smoke test for concurrent append/replace output patterns with threads.
marimo/_smoke_tests/status/status.py	Adds status component smoke coverage (progress bars/spinners).
marimo/_smoke_tests/output.py	Updates output smoke test formatting + includes replace-at-index usage.
marimo/_runtime/threads.py	Shares parent ExecutionContext.output into thread execution contexts.
marimo/_runtime/runner/hooks_post_execution.py	Adjusts output broadcast condition to use CellOutputList truthiness.
marimo/_runtime/output/_output.py	Refactors imperative output ops to use CellOutputList methods and stack().
marimo/_runtime/context/types.py	Makes ExecutionContext.output a non-optional CellOutputList via default_factory.
marimo/_runtime/cell_output_list.py	New thread-safe output container implementation.
marimo/_runtime/app/kernel_runner.py	Updates cached-output handling to stack CellOutputList.
marimo/_plugins/stateless/status/_progress.py	Adds locking around progress updates for thread-safety.

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[akshayka]

@coastalwhite this should generally improve your experience of writing to output from multiple threads (including with progress bars)