Inject tab drag capability registries

[austinywang]

Summary

• remove the hidden process-wide TabDragTransferRegistry singleton
• make default controllers own isolated registries
• expose explicit registry injection for hosts that exchange tabs across controllers

Testing

• commit d48c582 adds the regression test first and demonstrates that default controllers incorrectly shared process state
• commit a4f2d28 makes the capability tests pass with explicit sharing and default isolation

Parent fix: manaflow-ai/cmux#9787
Issue: manaflow-ai/cmux#9521

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Summary by cubic

Removes the hidden process-wide tab drag registry and isolates controllers by default. Adds an initializer to share a registry when controllers need to exchange tabs (fixes manaflow-ai/cmux#9521).

• Refactors

  • Removed TabDragTransferRegistry.process.
  • SplitViewController now creates a fresh TabDragTransferRegistry by default.
  • Added BonsplitController(configuration:tabDragTransferRegistry:) for explicit registry sharing.

• Migration

  • For cross-controller tab exchange, create one TabDragTransferRegistry() and pass it to each controller via BonsplitController(configuration:tabDragTransferRegistry:).
  • Keep using BonsplitController(configuration:) for isolated controllers.
  • Replace any .process references with an explicit TabDragTransferRegistry() instance.

^{Written for commit a4f2d28. Summary will update on new commits.}

Summary by CodeRabbit

• New Features

  • Controllers can now explicitly share a tab-drag transfer registry for coordinated drag-and-drop across controllers.
  • Default controllers use isolated tab-drag capabilities automatically.
  • Registry instances remain publicly constructible for customized sharing.

• Bug Fixes

  • Prevented unintended process-wide sharing of tab-drag transfer capabilities.

[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: 05077a18-329a-4c19-bef1-b1315307bcd9

📥 Commits

Reviewing files that changed from the base of the PR and between 882bd69 and a4f2d28.

📒 Files selected for processing (4)

• Sources/Bonsplit/Internal/Controllers/SplitViewController.swift
• Sources/Bonsplit/Public/BonsplitController.swift
• Sources/Bonsplit/Public/TabDragTransferRegistry.swift
• Tests/BonsplitTests/TabDragTransferRegistryTests.swift

💤 Files with no reviewable changes (1)

• Sources/Bonsplit/Public/TabDragTransferRegistry.swift

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📝 Walkthrough

Walkthrough

The change removes the process-wide tab-drag registry. BonsplitController now exposes explicit registry injection, while default controllers create isolated registries. Tests cover shared cross-controller routing and registry isolation.

Changes

Tab-drag registry ownership

Layer / File(s)	Summary
Registry API and controller wiring Sources/Bonsplit/Public/TabDragTransferRegistry.swift, Sources/Bonsplit/Public/BonsplitController.swift, Sources/Bonsplit/Internal/Controllers/SplitViewController.swift	Removes TabDragTransferRegistry.process. Makes registry injection public and explicit. Default controllers create new registry instances.
Registry ownership tests Tests/BonsplitTests/TabDragTransferRegistryTests.swift	Updates cross-controller drag coverage to inject one shared registry. Adds coverage confirming default controllers use distinct registries.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• manaflow-ai/bonsplit#202: Both changes modify tab-drag registry ownership and injection for cross-pane drop handling.

Suggested reviewers: lawrencecchen

🚥 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 clearly describes the primary change: explicit injection of tab drag capability registries.
Linked Issues check	✅ Passed	The changes implement isolated default registries and explicit shared registry injection, satisfying the linked issue’s tab-drag state requirement [#9787].
Out of Scope Changes check	✅ Passed	The changes are limited to registry ownership, dependency injection, singleton removal, and related regression tests.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-9521-injected-tab-drag-registry

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