ci: preserve output activity heartbeats in GhosttyKit

[onliner10]

What

Hey @thdxg - following up on #3. We discussed carrying the accepted output_activity change as .github/downstream/0002-output-activity.patch, but while finishing the Macterm integration I noticed that 0002 never actually made it into main or the latest GhosttyKit release. No worries - it looks like the follow-up simply slipped through, so I’m submitting the downstream-patch integration here.

This PR:

• preserves the GHOSTTY_ACTION_OUTPUT_ACTIVITY API as ordered patch 0002, retaining the original authorship and date;
• adds it to the sync and drift-check preservation lists;
• makes both workflows fail loudly if a declared downstream file is missing;
• updates the build workflow to Zig 0.16, matching Ghostty’s current minimum version.

The patch includes the original IO-path heartbeat from #3 plus the hardening found during Macterm integration: post-parse scrollbar geometry, no heartbeat for empty manual-API buffers, immediate retry after a full surface mailbox, stable appended enum values, and explicit native macOS/GTK no-ops for the embedder-only action.

Why

Macterm now uses this heartbeat to track in-place terminal activity, including same-PID Pi commands and fully occluded tabs. The renderer-driven scrollbar signal stops under occlusion, while this PTY IO-path signal continues to report real output.

Keeping it as a downstream patch ensures the daily pristine-upstream sync cannot discard it.

Verified

• Replayed 0001 then 0002 cleanly from the current upstream base.
• Verified the replayed tree exactly matches the tested source tree.
• Ran Zig formatting, focused output-activity/API tests, and a full non-app Zig build.
• Ran the native macOS Xcode build and tests directly.
• Built the final universal GhosttyKit XCFramework with Zig 0.16.
• Swapped that framework into Macterm and verified occluded Pi activity, physical and programmatic input, blank-Return/TUI false-positive guards, canonical foreground commands, and no resurrection after completion.

AI disclosure

The original implementation in #3 used Claude Code, as disclosed there. This downstream adaptation and review used OpenAI ChatGPT through Pi for code analysis, test design, and patch/workflow edits. I directed the work, reviewed the final diff, and ran the verification above.

Summary by CodeRabbit

• New Features

  • Added an output activity signal for embedders when terminal output is produced.
  • Signals include current scrollbar information and continue being delivered when the terminal surface is not visible.
  • Activity updates are throttled to avoid excessive notifications, while empty output is ignored.

• Chores

  • Updated build automation to use Zig 0.16.0.
  • Improved validation for downstream patch files during automated workflows.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 1360ccf8-6b9a-49b3-85c5-ca0ce4055ca6

📥 Commits

Reviewing files that changed from the base of the PR and between 2da9377 and a5f8293.

📒 Files selected for processing (4)

• .github/downstream/0002-output-activity.patch
• .github/workflows/assert-no-drift.yml
• .github/workflows/build-ghosttykit.yml
• .github/workflows/sync-upstream.yml

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📝 Walkthrough

Walkthrough

Adds a throttled PTY output activity heartbeat carrying scrollbar geometry through the surface mailbox and app action path, exposes it in public action APIs, adds platform no-op handling, and strengthens downstream workflow file validation while updating Zig.

Changes

Output activity heartbeat

Layer / File(s)	Summary
Activity action and message contracts .github/downstream/0002-output-activity.patch	Adds the output_activity action and scrollbar payload to the C ABI, Zig action union, and surface message union.
PTY output emission and throttling .github/downstream/0002-output-activity.patch	Emits non-empty PTY output activity at most every 500ms, preserves retry eligibility when mailbox delivery is coalesced, and tests empty-buffer and throttle behavior.
Surface delivery and platform dispatch .github/downstream/0002-output-activity.patch	Forwards scrollbar geometry through rt_app.performAction; macOS and GTK treat the action as an embedder-only no-op.

Downstream workflow maintenance

Layer / File(s)	Summary
Downstream patch tracking and validation .github/workflows/assert-no-drift.yml, .github/workflows/sync-upstream.yml	Tracks 0002-output-activity.patch and fails when declared downstream files are missing.
GhosttyKit Zig version .github/workflows/build-ghosttykit.yml	Updates the workflow’s Zig version from 0.15.2 to 0.16.0.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Termio
  participant SurfaceMailbox
  participant Surface
  participant App
  Termio->>SurfaceMailbox: Queue throttled output_activity with scrollbar geometry
  SurfaceMailbox->>Surface: Deliver output_activity message
  Surface->>App: Call rt_app.performAction

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: preserving GhosttyKit output-activity heartbeats.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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_{Comment @coderabbitai help to get the list of available commands.}

[thdxg]

@onliner10 Thanks for your work! It looks good to me.