macOS: dragging a title-bar control no longer moves the window

AppKit moves the window when a drag starts anywhere in the title-bar band
of a full-size content view, on the application bar's controls too. The
title bar now reports whether the pointer is on one of its controls
(title_bar::window_movable), and the macOS app sets the window's movable
flag from it each frame it changes, through objc2-app-kit's safe methods
(the versions muda already uses). Home, the menus, search, the workspace
menu, Share and Discord keep their drags; empty bar space still moves the
window, and so does the band while a modal covers the bar.

Cargo.lock

@@ -878,6 +878,8 @@ dependencies = [
  "egui",
  "log",
  "muda",
+ "objc2 0.6.4",
+ "objc2-app-kit 0.3.2",
  "rfd",
  "serde",
  "serde_json",

apps/designcraft/Cargo.toml

@@ -31,3 +31,6 @@ winit = { version = "0.30", default-features = false, features = ["x11"] }
 
 [target.'cfg(target_os = "macos")'.dependencies]
 muda = { version = "0.21", default-features = false }
+# The versions muda uses: the window's `movable` flag (see `mac_window`), through safe methods.
+objc2 = { version = "0.6", default-features = false, features = ["std"] }
+objc2-app-kit = { version = "0.3", default-features = false, features = ["std", "NSApplication", "NSResponder", "NSWindow"] }

apps/designcraft/src/mac_window.rs

@@ -0,0 +1,31 @@
+//! macOS: AppKit moves the window when a drag starts anywhere in the title-bar band, on the
+//! application bar's controls too. The window is made unmovable while the pointer is on one of
+//! them (`designcraft_ui_egui::title_bar::window_movable`), so only the bar's empty space moves it.
+
+use objc2::MainThreadMarker;
+use objc2_app_kit::{NSApplication, NSWindowStyleMask};
+
+/// Sets the window's `movable` flag from this frame's title bar. `applied` holds the value last
+/// set, so AppKit is only called when it changes.
+pub fn sync_movable(ctx: &egui::Context, applied: &mut Option<bool>) {
+    let movable = designcraft_ui_egui::title_bar::window_movable(ctx);
+    if *applied == Some(movable) {
+        return;
+    }
+    // eframe runs the app on the main thread; anywhere else AppKit is out of reach.
+    let Some(mtm) = MainThreadMarker::new() else {
+        return;
+    };
+    let mut found = false;
+    for window in NSApplication::sharedApplication(mtm).windows().to_vec() {
+        // The main window is the only one with a full-size content view.
+        if window.styleMask().contains(NSWindowStyleMask::FullSizeContentView) {
+            window.setMovable(movable);
+            found = true;
+        }
+    }
+    // Before the window is on screen there is nothing to set yet; try again next frame.
+    if found {
+        *applied = Some(movable);
+    }
+}

apps/designcraft/src/main.rs

@@ -15,6 +15,8 @@ mod control_server;
 mod gpu;
 mod logging;
 #[cfg(target_os = "macos")]
+mod mac_window;
+#[cfg(target_os = "macos")]
 mod native_menu;
 
 use designcraft_engine::Session;
@@ -24,6 +26,9 @@ struct App {
     app: DesignApp,
     #[cfg(target_os = "macos")]
     menu: Option<native_menu::NativeMenu>,
+    /// The window's `movable` flag as last set; see `mac_window`.
+    #[cfg(target_os = "macos")]
+    movable: Option<bool>,
     /// The graphics start in progress (`gpu.json`) until a frame has been presented; see `gpu`.
     startup: Option<gpu::Startup>,
     /// Frames begun so far.
@@ -57,6 +62,8 @@ impl eframe::App for App {
     }
     fn ui(&mut self, ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
         self.app.ui(ui);
+        #[cfg(target_os = "macos")]
+        mac_window::sync_movable(ui.ctx(), &mut self.movable);
     }
     fn on_exit(&mut self) {
         // A normal exit before the third frame is not a crash.
@@ -332,6 +339,8 @@ fn main() -> eframe::Result {
                 app,
                 #[cfg(target_os = "macos")]
                 menu: None,
+                #[cfg(target_os = "macos")]
+                movable: None,
                 startup: gpu,
                 frames: 0,
             }))

crates/ui-egui/src/chrome.rs

@@ -942,6 +942,30 @@ mod tests {
         }
     }
 
+    #[test]
+    fn the_mac_window_is_not_movable_on_the_title_bar_controls() {
+        use super::{MAC_APP_BAR, MAC_CONTENT_LEFT};
+        let mut app = app();
+        app.integrated_titlebar = true;
+        let mut h = test_window::open(app, vec2(1400.0, 900.0));
+        let bar = test_window::panel_rect(&h, "app_bar");
+        let mid = bar.min.y + MAC_APP_BAR / 2.0;
+        // The search field is the bar's last control: 125 wide, 4 from the 10-wide right margin.
+        let search = pos2(bar.max.x - 10.0 - 4.0 - 60.0, mid);
+        let controls = [rect(&h, "Home").center(), rect(&h, "Share").center(), rect(&h, "File").center(), search];
+        let mut movable_at = |at: egui::Pos2| {
+            h.hover_at(at);
+            h.run_steps(2);
+            crate::title_bar::window_movable(&h.ctx)
+        };
+        for at in [pos2(bar.center().x, mid), pos2(bar.min.x + MAC_CONTENT_LEFT - 4.0, bar.min.y + 4.0), pos2(bar.center().x, 500.0)] {
+            assert!(movable_at(at), "empty title bar or the page at {at:?}");
+        }
+        for at in controls {
+            assert!(!movable_at(at), "control at {at:?}");
+        }
+    }
+
     #[test]
     fn a_narrow_window_scrolls_the_app_bar() {
         let w = 640.0;

crates/ui-egui/src/title_bar.rs

@@ -2,8 +2,10 @@
 //! when dragged and follows the system's title-bar double-click setting.
 //!
 //! The window has a full-size content view, so AppKit hands every click in the title-bar band to
-//! the app (that is how the bar's own controls work) and neither drags nor zooms the window
-//! itself.
+//! the app (that is how the bar's own controls work) and does not zoom the window. It does move
+//! the window when a drag starts anywhere in the band, on the bar's controls too, unless the
+//! window is not movable. [`window_movable`] tells the macOS shell when the pointer is on a
+//! control, so it can switch that off before the press.
 
 use std::sync::{Once, OnceLock};
 
@@ -70,6 +72,12 @@ pub fn double_click_command(action: DoubleClick, maximized: bool) -> Option<egui
 pub fn empty_area(ui: &mut egui::Ui, rect: egui::Rect) {
     prefetch();
     let resp = ui.interact(rect, ui.id().with("title_bar_empty"), egui::Sense::click());
+    // A control in this layer covers the empty area under the pointer. Another layer on top (a
+    // modal's backdrop, a tooltip) leaves the window movable, as AppKit's own title bar would be.
+    let on_control =
+        ui.ctx().pointer_hover_pos().is_some_and(|p| rect.contains(p) && ui.ctx().layer_id_at(p) == Some(ui.layer_id())) && !resp.hovered();
+    let pass = ui.ctx().cumulative_pass_nr();
+    ui.data_mut(|d| d.insert_temp(movable_id(), (pass, !on_control)));
     if resp.double_clicked() {
         let maximized = ui.input(|i| i.viewport().maximized).unwrap_or(false);
         if let Some(cmd) = double_click_command(action(), maximized) {
@@ -91,6 +99,20 @@ pub fn empty_area(ui: &mut egui::Ui, rect: egui::Rect) {
     }
 }
 
+fn movable_id() -> egui::Id {
+    egui::Id::new("title_bar_window_movable")
+}
+
+/// Whether AppKit may move the window when a drag starts in the title-bar band: false while the
+/// pointer is on one of the bar's controls, so pressing and dragging it does not move the window.
+/// True when neither this pass nor the one before drew a title bar.
+pub fn window_movable(ctx: &egui::Context) -> bool {
+    match ctx.data(|d| d.get_temp::<(u64, bool)>(movable_id())) {
+        Some((pass, movable)) if ctx.cumulative_pass_nr().saturating_sub(pass) <= 1 => movable,
+        _ => true,
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;