macOS: linger before stopping a display link to stop io-thread churn

[neo773]

Summary

• Every CVDisplayLinkStart spawns a fresh io thread. Windows unsubscribe from their display's link whenever frame requests park (pause_frame_requests). A timer-driven animation, like a 30 Hz spinner, parks between every frame, so the link was stopped and restarted once per frame: one new thread per frame.
• A display's link now keeps running for 250 ms (STOP_LINGER) after its last subscriber leaves:
  • A subscriber that returns within that window reuses the running link, with no restart and no new thread.
  • Idle windows still stop the link shortly after parking.
  • While the link lingers, its output callback finds no subscribers and does nothing.
• The running/stop logic moves into a small LinkState:
  • A generation counter makes a deferred stop a no-op if anyone subscribed after it was scheduled.
  • The deferred stop runs on the main queue, so the registry's main-thread-only rule and lock ordering are unchanged.

Measurements

A probe window redrawing on a 30 Hz timer, sampled for 3 s with /usr/bin/sample and counting CVDisplayLink threads:

	Run 1	Run 2	Run 3
Before	89	88	87
After	1	1	1

In a real app (Zeron) with a sidebar spinner running, a 2 s sample showed 25 CVDisplayLink threads come and go.

Test plan

• cargo test -p gpui_macos --lib display_link: 5 new LinkState tests covering:
  • starting the link once
  • stopping after the linger
  • resubscribing during the linger without a restart
  • only the latest deferred stop applying
  • no stop scheduled when the link never ran
• cargo clippy -p gpui_macos: no warnings in display_link.rs
• The 30 Hz probe above, before and after the change

🤖 Generated with Claude Code

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