fix(shell): throttle text output updates to prevent UI jank

[ixchio]

Quick one — the UI freezes up during high-volume shell output.

What's happening

Every text data event triggers a React re-render. Run something like npm test on a big project or a build with hundreds of warnings, and the UI becomes completely unresponsive. Meanwhile, binary_progress already throttles to OUTPUT_UPDATE_INTERVAL_MS (1 second) — text output just... didn't get the same treatment.

The fix

Applied the same Date.now() - lastUpdateTime > OUTPUT_UPDATE_INTERVAL_MS time-gate to the data case in two places:

1. packages/core/src/tools/shell.ts — the tool execution path
2. packages/cli/src/ui/hooks/useExecutionLifecycle.ts — the user shell (! prefix) path

The final output is always delivered via the result object after the command completes, so you never lose anything — you just don't re-render 500 times a second while it's running.

Why not PR #25461?

That PR was flagged for unbounded memory growth in cumulativeOutput accumulation. This fix doesn't change what gets accumulated — it only gates how often we call updateOutput() / setPendingHistoryItem(). The accumulation behavior stays exactly the same.

Total diff is like 8 lines.

Fixes #25459

cc @NTaylorMullen @abhipatel12 this touches your area — thoughts?

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses UI unresponsiveness caused by high-frequency shell output updates. By introducing a time-gated check for text data events, the application avoids unnecessary re-renders while maintaining the integrity of the final output once command execution completes.

Highlights

• UI Performance Optimization: Implemented a time-based throttle for shell text output updates to prevent excessive React re-renders during high-volume data streams.
• Consistent Throttling Logic: Applied the existing OUTPUT_UPDATE_INTERVAL_MS gate to the 'data' event handlers in both the core shell tool and the CLI execution lifecycle hook.

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[gemini-code-assist]

Code Review

This pull request implements throttling for UI updates during command execution to prevent excessive re-renders. While the approach addresses performance, the review highlights critical issues: initializing the update timer to the current time causes a delay in showing the first output, binary progress events are missing throttling, and the shell tool logic leads to data loss because output chunks are overwritten rather than accumulated during the throttle window.

[Jordanwaslistening]

Code Review

This pull request implements throttling for UI updates during command execution to prevent excessive re-renders. While the approach addresses performance, the review highlights critical issues: initializing the update timer to the current time causes a delay in showing the first output, binary progress events are missing throttling, and the shell tool logic leads to data loss because output chunks are overwritten rather than accumulated during the throttle window.