surface.read_text scrollback readback runs entirely on main thread, can starve UI under heavy agent workloads

[arzafran]

Tracking upstream cmux manaflow-ai#6500 — applicable to our fork but needs a product/scope decision.

Assessment: Confirmed: v2SurfaceReadText at Sources/TerminalController.swift:5808 wraps the entire read in v2MainSync, and readTerminalTextBase64 calls ghostty_surface_read_text three times on main when includeScrollback=true. CLAUDE.md socket threading policy explicitly lists surface.read_text as a main-thread command due to snapshot-consistency requirements. Moving the heavy C call off-main requires first verifying ghostty_surface_read_text thread-safety in the Ghostty Zig implementation. Needs both a thread-safety audit and a policy decision.

Upstream: manaflow-ai#6500

[arzafran]

Decision: Defer (needs audit). Keep surface.read_text on main for snapshot consistency (per the socket threading policy). Moving the heavy ghostty_surface_read_text call off-main is blocked on a thread-safety audit of the Ghostty Zig implementation. Cleared needs-decision; revisit after the audit.

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Generated by Claude Code

[arzafran]

Closing as deferred tech-debt. surface.read_text scrollback readback on the main thread is a real latent cost but has not been observed to starve the UI in practice. Revisit if profiling shows it materially blocks the main actor under heavy agent workloads; the fix (off-main readback seam) can be reopened then.