feat(macos): tag the Metal layer with the display colorspace on demand

When the app self-manages the display transform (OAK_MACOS_LAYER_
COLORSPACE=display set at startup), the CAMetalLayer is tagged with the
main display's colorspace so ColorSync passes the already-transformed
pixels through — without the tag the OS would re-correct them (double
correction). The CoreGraphics FFI lives in its own module because the
build script forwards metal_renderer.rs FFI declarations into the
Metal shader header.

crates/gpui_macos/src/display_colorspace.rs

@@ -0,0 +1,39 @@
+// GPUI macOS platform - GPUI is licensed under the Apache License, Version 2.0
+// (see the gpui submodule's license).
+
+//! The main display's colorspace as a raw `CGColorSpaceRef`, for the
+//! color-management coordination in `metal_renderer` (when the app
+//! self-manages the display transform, the CAMetalLayer is tagged with the
+//! display's colorspace so ColorSync passes the pixels through).
+//!
+//! This lives in its own module because the `metal_renderer` build script
+//! scans `metal_renderer.rs` for FFI declarations and forwards them into
+//! the Metal shader header — raw extern blocks must not be added there.
+
+use std::ffi::c_void;
+
+#[link(name = "CoreGraphics", kind = "framework")]
+unsafe extern "C" {
+	fn CGMainDisplayID() -> u32;
+	fn CGDisplayCopyColorSpace(display: u32) -> *mut c_void;
+}
+
+#[link(name = "CoreFoundation", kind = "framework")]
+unsafe extern "C" {
+	fn CFRelease(obj: *const c_void);
+}
+
+/// The main display's colorspace as a raw pointer (caller's
+/// responsibility to keep it alive for the msg_send call; released here
+/// after the layer call returns — the layer retains it). `None` when the
+/// display has no colorspace (headless).
+pub fn with_main_display_colorspace(f: impl FnOnce(*mut c_void)) {
+	unsafe {
+		let space = CGDisplayCopyColorSpace(CGMainDisplayID());
+		if space.is_null() {
+			return;
+		}
+		f(space);
+		CFRelease(space);
+	}
+}

crates/gpui_macos/src/gpui_macos.rs

@@ -6,6 +6,7 @@
 
 mod dispatcher;
 mod display;
+mod display_colorspace;
 mod display_link;
 mod events;
 mod haptic_feedback;

crates/gpui_macos/src/metal_renderer.rs

@@ -251,6 +251,20 @@ impl MetalRenderer {
 			];
 		}
 
+		// Color management coordination: when the app self-manages the
+		// display transform (OAK_MACOS_LAYER_COLORSPACE=display, set at
+		// startup when display-ICC color management is active), tag the
+		// layer with the DISPLAY's colorspace so ColorSync's mapping
+		// becomes a pass-through — otherwise the OS would re-correct our
+		// already-corrected pixels (double correction).
+		if std::env::var_os("OAK_MACOS_LAYER_COLORSPACE").as_deref()
+			== Some(std::ffi::OsStr::new("display"))
+		{
+			crate::display_colorspace::with_main_display_colorspace(|space| unsafe {
+				let _: () = msg_send![&*layer, setColorspace: space];
+			});
+		}
+
 		Self::new_internal(device, Some(layer), !transparent, instance_buffer_pool)
 	}