Add eframe::WindowChromeMetrics (macOS only) (#8015)

When using `egui::ViewportBuilder::with_fullsize_content_view` one must
be careful not to paint anything where the "traffic light" buttons are:

<img width="87" height="47" alt="image"
src="https://github.com/user-attachments/assets/0e878c8e-7141-4fed-bbc8-4d542ddb5251"
/>

`eframe::WindowChromeMetrics` helps you with that!

crates/eframe/src/lib.rs

@@ -190,6 +190,9 @@ pub use web::{WebLogger, WebRunner};
 #[cfg(any(feature = "glow", feature = "wgpu_no_default_features"))]
 mod native;
 
+#[cfg(target_os = "macos")]
+pub use native::macos::WindowChromeMetrics;
+
 #[cfg(not(target_arch = "wasm32"))]
 #[cfg(any(feature = "glow", feature = "wgpu_no_default_features"))]
 pub use native::run::EframeWinitApplication;

crates/eframe/src/native/macos.rs

@@ -0,0 +1,76 @@
+use egui::Vec2;
+use objc2_app_kit::{NSView, NSWindow, NSWindowButton};
+use raw_window_handle::{AppKitWindowHandle, RawWindowHandle};
+
+/// Size of the "traffic lights" (red/yellow/green close/minimize/maximize buttons)
+/// on the native macOS window.
+///
+/// This is very useful together with [`egui::ViewportBuilder::with_fullsize_content_view`].
+#[derive(Debug)]
+pub struct WindowChromeMetrics {
+    /// Size of the "traffic lights" (red/yellow/green close/minimize/maximize buttons),
+    /// including margins.
+    ///
+    /// The unit here is in "native scale", which means it needs to be divided by [`egui::Context::zoom_factor`]
+    /// to get the size in egui points.
+    pub traffic_lights_size: Vec2,
+}
+
+impl WindowChromeMetrics {
+    /// Get the window chrome metrics for a given window handle.
+    pub fn from_window_handle(window_handle: &RawWindowHandle) -> Option<Self> {
+        window_chrome_metrics(window_handle)
+    }
+}
+
+fn window_chrome_metrics(window_handle: &RawWindowHandle) -> Option<WindowChromeMetrics> {
+    let RawWindowHandle::AppKit(appkit_handle) = window_handle else {
+        return None;
+    };
+
+    let ns_view = ns_view_from_handle(appkit_handle)?;
+    let ns_window = ns_view.window()?;
+
+    Some(WindowChromeMetrics {
+        traffic_lights_size: traffic_lights_metrics(&ns_window)?,
+    })
+}
+
+fn traffic_lights_metrics(ns_window: &NSWindow) -> Option<Vec2> {
+    // Button order is CloseButton, MiniaturizeButton, ZoomButton:
+    let close_button = ns_window
+        .standardWindowButton(NSWindowButton::CloseButton)?
+        .frame();
+    let zoom_button = ns_window
+        .standardWindowButton(NSWindowButton::ZoomButton)?
+        .frame();
+
+    let left_margin = close_button.origin.x;
+    let right_margin = left_margin; // for symmetry
+
+    let total_width = zoom_button.origin.x + zoom_button.size.width + right_margin;
+
+    let top_margin = close_button.origin.y;
+    let bottom_margin = top_margin; // Usually symmetric
+    let total_height = top_margin + close_button.size.height + bottom_margin;
+
+    Some(Vec2::new(total_width as f32, total_height as f32))
+}
+
+fn ns_view_from_handle(handle: &AppKitWindowHandle) -> Option<&NSView> {
+    let ns_view_ptr = handle.ns_view.as_ptr().cast::<NSView>();
+
+    // Validate the pointer is non-null
+    if ns_view_ptr.is_null() {
+        None
+    } else {
+        // SAFETY:
+        // - We've verified the pointer is non-null
+        // - The pointer comes from the windowing system, so it should be valid
+        // - NSView pointers from AppKit are expected to remain valid for the window lifetime
+        #[expect(unsafe_code)]
+        unsafe {
+            ns_view_ptr.as_ref()
+        }
+    }
+}

crates/eframe/src/native/mod.rs

@@ -3,6 +3,9 @@ mod epi_integration;
 mod event_loop_context;
 pub mod run;
 
+#[cfg(target_os = "macos")]
+pub(crate) mod macos;
+
 /// File storage which can be used by native backends.
 #[cfg(feature = "persistence")]
 pub mod file_storage;