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# Objective Prevent a macOS crash in GPUI when AppKit temporarily reports that a visible window has no associated `NSScreen` during display reconfiguration. The crash can happen on this path: ```text NSScreen::deviceDescription gpui_macos:🪟:display_id_for_screen gpui_macos:🪟:MacWindowState::start_display_link gpui_macos:🪟:window_did_change_screen ``` `start_display_link` checked the window occlusion state before creating a display link, but it still assumed `NSWindow.screen()` was non-null. During display changes, sleep/wake, lid close/open, or monitor reconfiguration, AppKit can transiently return `nil` for `screen`, and passing that into `NSScreen::deviceDescription` aborts with a null pointer dereference. I observed this on macOS 26.5 after the system entered a rare display state: once in that state, running GPUI applications would crash immediately after wake. I have not identified the exact OS/display condition that triggers it, so the full wake/reconfiguration scenario is not deterministic, but the crash report consistently points to `NSWindow.screen()` being `nil` when `start_display_link` tries to create a display link. ## Solution Make `display_id_for_screen` explicitly handle a null `NSScreen` by returning `None`. Callers now handle that case by either: - returning early from `start_display_link`, because there is no valid display id to create a display link for yet - skipping a null screen while iterating `NSScreen::screens` The normal non-null screen path is unchanged. This keeps the nil-screen guard at the FFI boundary where `NSScreen::deviceDescription` is called. ## Testing Tested on macOS 26.5. Commands run: ```bash cargo fmt --check -p gpui_macos cargo test -p gpui_macos display_id_for_screen_returns_none_for_null_screen cargo check -p gpui_macos ``` Added a unit test covering the new boundary behavior: `window::tests::display_id_for_screen_returns_none_for_null_screen` ## Self-Review Checklist: - [x] I've reviewed my own diff for quality, security, and reliability - [x] Unsafe blocks (if any) have justifying comments - [x] The content adheres to Zed's UI standards ([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist) and [icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md) guidelines) - [x] Tests cover the new/changed behavior - [x] Performance impact has been considered and is acceptable --- Release Notes: - Fixed a macOS crash that could occur when display configuration changes while a GPUI window is temporarily not associated with a screen. |
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