fix(window): stop macOS traffic lights jittering on resize

AppKit puts the title bar container and window buttons back at their defaults on every
live resize step, title change and fullscreen exit. Correcting them from window notifications
ran after that layout had been drawn, so the buttons bounced between the two positions, and
stayed at the defaults when no notification followed.

Correct the frames in a main run loop observer that runs after AppKit's layout and before
the Core Animation commit. Create project windows hidden so the buttons are placed before the
first frame, and skip repeated height updates from the toolbar.

src-tauri/src/titlebar.rs

@@ -5,19 +5,26 @@ use std::collections::HashMap;
 use std::rc::Rc;
 
 use block2::RcBlock;
-use objc2::rc::Retained;
-use objc2::runtime::ProtocolObject;
-use objc2_app_kit::{NSWindow, NSWindowButton, NSWindowDidUpdateNotification, NSWindowStyleMask};
-use objc2_foundation::{NSNotificationCenter, NSObjectProtocol, NSPoint};
+use objc2_app_kit::{NSWindow, NSWindowButton, NSWindowStyleMask};
+use objc2_core_foundation::{
+    CFIndex, CFOptionFlags, CFRetained, CFRunLoop, CFRunLoopActivity, CFRunLoopObserver, kCFRunLoopCommonModes,
+};
+use objc2_foundation::NSPoint;
+
+/// AppKit lays out the title bar in a run loop observer and puts the buttons back at their
+/// defaults on resize, title changes and fullscreen exit. Core Animation commits at 2000000, so
+/// correcting the frames just before it keeps AppKit's positions off screen.
+const BEFORE_COMMIT: CFIndex = 1_999_999;
 
 struct Titlebar {
     height: Rc<Cell<f64>>,
-    observer: Retained<ProtocolObject<dyn NSObjectProtocol>>,
+    spacing: f64,
+    observer: CFRetained<CFRunLoopObserver>,
 }
 
 impl Drop for Titlebar {
     fn drop(&mut self) {
-        unsafe { NSNotificationCenter::defaultCenter().removeObserver((*self.observer).as_ref()) };
+        self.observer.invalidate();
     }
 }
 
@@ -30,22 +37,30 @@ pub fn set_height(window: &tauri::WebviewWindow, height: f64) -> tauri::Result<(
     // with_webview runs on the AppKit thread; the observer is also delivered there.
     window.with_webview(move |webview| unsafe {
         let native = &*webview.ns_window().cast::<NSWindow>();
+        let Some(close) = native.standardWindowButton(NSWindowButton::CloseButton) else { return };
+        let Some(minimize) = native.standardWindowButton(NSWindowButton::MiniaturizeButton) else { return };
+        let Some(run_loop) = CFRunLoop::main() else { return };
+        let spacing = minimize.frame().origin.x - close.frame().origin.x;
         TITLEBARS.with_borrow_mut(|bars| {
-            let bar = bars.entry(label).or_insert_with(|| {
-                let height = Rc::new(Cell::new(height));
-                let current = height.clone();
-                let pointer = native as *const NSWindow;
-                let block = RcBlock::new(move |_| center_buttons(&*pointer, current.get()));
-                let observer = NSNotificationCenter::defaultCenter().addObserverForName_object_queue_usingBlock(
-                    Some(NSWindowDidUpdateNotification),
-                    Some(native),
-                    None,
-                    &block,
-                );
-                Titlebar { height, observer }
+            if let Some(bar) = bars.get(&label) {
+                bar.height.set(height);
+                center_buttons(native, height, bar.spacing);
+                return;
+            }
+            let shared = Rc::new(Cell::new(height));
+            let current = shared.clone();
+            let pointer = native as *const NSWindow;
+            let block = RcBlock::new(move |_: *mut CFRunLoopObserver, _: CFRunLoopActivity| {
+                center_buttons(&*pointer, current.get(), spacing)
             });
-            bar.height.set(height);
-            center_buttons(native, height);
+            let activities = CFRunLoopActivity::BeforeWaiting.0 as CFOptionFlags;
+            let Some(observer) = CFRunLoopObserver::with_handler(None, activities, true, BEFORE_COMMIT, Some(&block))
+            else {
+                return;
+            };
+            run_loop.add_observer(Some(&observer), kCFRunLoopCommonModes);
+            center_buttons(native, height, spacing);
+            bars.insert(label, Titlebar { height: shared, spacing, observer });
         });
     })
 }
@@ -57,14 +72,12 @@ pub fn forget(window: &tauri::Window) {
     });
 }
 
-unsafe fn center_buttons(window: &NSWindow, height: f64) {
+unsafe fn center_buttons(window: &NSWindow, height: f64, spacing: f64) {
     // Fullscreen uses AppKit's own revealable title bar.
     if window.styleMask().contains(NSWindowStyleMask::FullScreen) {
         return;
     }
     let Some(close) = window.standardWindowButton(NSWindowButton::CloseButton) else { return };
-    let Some(minimize) = window.standardWindowButton(NSWindowButton::MiniaturizeButton) else { return };
-    let spacing = minimize.frame().origin.x - close.frame().origin.x;
     let Some(container) = (unsafe { close.superview().and_then(|view| view.superview()) }) else { return };
     let Some(content) = window.contentView() else { return };
     let mut frame = container.frame();

src-tauri/src/windows.rs

@@ -284,6 +284,11 @@ fn hex_color(hex: &str) -> Option<Color> {
 /// Builds a project window painted in the saved theme's frame colour, so it
 /// doesn't flash the light default before its page (and splash) loads.
 fn build_project_window(app: &AppHandle, mut config: WindowConfig) -> IdeResult<WebviewWindow> {
+    let visible = cfg!(target_os = "macos") && config.visible;
+    let focused = config.focus;
+    if visible {
+        config.visible = false;
+    }
     let state = app.try_state::<AppState>();
     let appearance = state.as_ref().map(|s| s.global_settings.effective().appearance);
     // A plugin theme replaces the built-in one in either OS mode; the UI records its resolved frame.
@@ -305,6 +310,12 @@ fn build_project_window(app: &AppHandle, mut config: WindowConfig) -> IdeResult<
     {
         let _ = window.set_background_color(Some(frame_color(&a, true)));
     }
+    if visible {
+        window.show().map_err(|e| IdeError::internal(format!("cannot show window: {e}")))?;
+        if focused {
+            let _ = window.set_focus();
+        }
+    }
     Ok(window)
 }