#968: centre the macOS window buttons on the app bar

AppKit places close, minimize and zoom in a standard title bar, shorter
than the 44-point app bar they sit over, so they rode high on it. They
are now moved to the middle of the bar, inset from its left as far as
from its top, with the title bar container made as tall as the bar so
they stay clickable. AppKit lays the title bar out again on resize and
when leaving full screen, so the placement runs every frame and sets a
frame only when it is out of place. The app bar keeps the room the
buttons take, measured, instead of a fixed 78 points.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/vectorcraft/Cargo.toml

@@ -51,10 +51,13 @@ windows = { version = "0.62", features = ["Win32_Graphics_DirectWrite"] }
 # with default features off). Pinned exactly: review the diff before bumping.
 muda = { version = "=0.21.1", default-features = false }
 objc2 = "0.6"
-objc2-app-kit = { version = "0.3", default-features = false, features = ["std", "NSApplication", "NSMenu", "NSEvent", "NSPasteboard", "NSResponder", "NSTextInputContext"] }
+objc2-app-kit = { version = "0.3", default-features = false, features = ["std", "NSApplication", "NSButton", "NSControl", "NSMenu", "NSEvent", "NSPasteboard", "NSResponder", "NSTextInputContext", "NSView", "NSWindow"] }
 # Files opened from Finder and the Dock (src/open_documents.rs) and pasted bitmaps (src/clipboard.rs); both already come with winit, muda and rfd.
-objc2-foundation = { version = "0.3", default-features = false, features = ["std", "block2", "NSArray", "NSData", "NSNotification", "NSOperation", "NSString", "NSURL"] }
+objc2-foundation = { version = "0.3", default-features = false, features = ["std", "block2", "NSArray", "NSData", "NSGeometry", "NSNotification", "NSOperation", "NSString", "NSURL"] }
 block2 = "0.6"
+# The window's AppKit view, to centre its buttons on the app bar (src/mac_window.rs). MIT OR
+# Apache-2.0 OR Zlib, already in the build through eframe and winit.
+raw-window-handle = "0.6"
 # CoreText's font manager (src/mac_fonts.rs): fonts that apps and font managers register from their
 # own folders (#579). Zlib OR Apache-2.0 OR MIT; objc2-core-foundation already comes with
 # objc2-foundation.

apps/vectorcraft/src/mac_window.rs

@@ -0,0 +1,74 @@
+//! macOS: the window buttons (close, minimize, zoom) centred on the app bar (#968).
+//!
+//! AppKit puts the buttons in a standard title bar, shorter than the app bar they sit over, so they
+//! rode high on it. [`center_buttons`] moves them to the middle of the bar, inset from its left as
+//! far as from its top, and makes the title bar as tall as the app bar so they stay clickable.
+//! AppKit lays its title bar out again when the window resizes or leaves full screen, so this runs
+//! every frame and moves the buttons only when they are out of place.
+//!
+//! Reaching the title bar's views from the window's content view has no safe API, hence the scoped
+//! `unsafe_code` allowance.
+#![allow(unsafe_code)]
+
+use objc2::runtime::AnyObject;
+use objc2_app_kit::{NSView, NSWindowButton, NSWindowStyleMask};
+use objc2_foundation::NSPoint;
+use raw_window_handle::{HasWindowHandle, RawWindowHandle};
+
+/// The room the app bar keeps at its left for the buttons in AppKit's own place, before they are
+/// centred (and in full screen, where AppKit shows them elsewhere): their width and a gap.
+pub const DEFAULT_INSET: f32 = 78.0;
+
+/// Differences below this (in points) leave a frame as it is, so a frame in place isn't set again.
+const SLACK: f64 = 0.25;
+
+/// Centre the window's buttons on an app bar `bar` egui points tall, at the window's `zoom` factor
+/// (native points per egui point). Returns the room the app bar keeps clear at its left for them,
+/// in egui points, or `None` when they aren't over the bar (full screen) or can't be found.
+pub fn center_buttons(frame: &eframe::Frame, zoom: f32, bar: f32) -> Option<f32> {
+    let RawWindowHandle::AppKit(handle) = frame.window_handle().ok()?.as_raw() else {
+        return None;
+    };
+    // SAFETY: the handle's view is the window's content view, which AppKit keeps alive while the
+    // window is open, and eframe hands out the handle only while it is.
+    let view = unsafe { handle.ns_view.cast::<NSView>().as_ref() };
+    let window = view.window()?;
+    if window.styleMask().contains(NSWindowStyleMask::FullScreen) {
+        return None;
+    }
+    let close = window.standardWindowButton(NSWindowButton::CloseButton)?;
+    let minimize = window.standardWindowButton(NSWindowButton::MiniaturizeButton)?;
+    let zoom_button = window.standardWindowButton(NSWindowButton::ZoomButton)?;
+    // The buttons sit in the title bar view, inside the title bar container that sets its height.
+    // SAFETY: both views belong to the window, alive while it is; used right away.
+    let container = unsafe { close.superview().and_then(|titlebar| titlebar.superview()) }?;
+    // Another layout (a later macOS) is left as AppKit made it.
+    if AnyObject::class(&container).name() != c"NSTitlebarContainerView" {
+        return None;
+    }
+    let size = close.frame().size;
+    let gap = minimize.frame().origin.x - (close.frame().origin.x + size.width);
+    let height = f64::from(bar * zoom);
+    let margin = (height - size.height) / 2.0;
+    if !(margin.is_finite() && margin > 0.0 && gap.is_finite() && gap >= 0.0 && zoom > 0.0) {
+        return None;
+    }
+    let mut bounds = container.frame();
+    let top = window.frame().size.height - height;
+    if (bounds.size.height - height).abs() > SLACK || (bounds.origin.y - top).abs() > SLACK {
+        bounds.size.height = height;
+        bounds.origin.y = top;
+        container.setFrame(bounds);
+    }
+    let mut x = margin;
+    for button in [close, minimize, zoom_button] {
+        let at = NSPoint::new(x, (height - button.frame().size.height) / 2.0);
+        let now = button.frame().origin;
+        if (now.x - at.x).abs() > SLACK || (now.y - at.y).abs() > SLACK {
+            button.setFrameOrigin(at);
+        }
+        x += size.width + gap;
+    }
+    // The last button's right edge (`x` steps one gap past it) and the margin again after it.
+    Some(((x - gap + margin) / f64::from(zoom)) as f32)
+}

apps/vectorcraft/src/main.rs

@@ -31,6 +31,8 @@ mod mac_fonts;
 #[cfg(target_os = "macos")]
 mod mac_menu;
 #[cfg(target_os = "macos")]
+mod mac_window;
+#[cfg(target_os = "macos")]
 mod open_documents;
 mod prefs_dir;
 mod printing;
@@ -69,7 +71,14 @@ impl eframe::App for App {
         }
         file_access::unconfined(|| self.app.raw_input_hook(raw));
     }
-    fn ui(&mut self, ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
+    #[cfg_attr(not(target_os = "macos"), allow(unused_variables))]
+    fn ui(&mut self, ui: &mut egui::Ui, frame: &mut eframe::Frame) {
+        #[cfg(target_os = "macos")]
+        {
+            let bar = vectorcraft_ui_egui::chrome::APP_BAR_HEIGHT;
+            let inset = mac_window::center_buttons(frame, ui.ctx().zoom_factor(), bar);
+            self.app.titlebar_inset = inset.unwrap_or(mac_window::DEFAULT_INSET);
+        }
         file_access::unconfined(|| self.app.ui(ui));
         #[cfg(target_os = "macos")]
         if self.app.take_ime_discard() {
@@ -566,7 +575,6 @@ fn main() -> std::process::ExitCode {
                 {
                     app.graphics_adapter = Some("OpenGL (Windows 7 compatibility)".into());
                 }
-                app.integrated_titlebar = cfg!(target_os = "macos");
                 app.custom_titlebar = CUSTOM_TITLEBAR;
                 if let Some(port) = control_port {
                     let rx = control_server::start(port, cc.egui_ctx.clone());