Title bar: window-gesture policy (drag, double-click zoom, full-screen exempt)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/ui-egui/src/lib.rs

@@ -25,6 +25,7 @@ pub mod softpaint;
 pub mod state;
 pub mod tasks;
 pub mod theme;
+pub mod titlebar;
 pub mod widgets;
 
 #[cfg(test)]

crates/ui-egui/src/titlebar.rs

@@ -0,0 +1,76 @@
+//! The integrated title bar's window gestures (macOS: the native title bar is hidden and the top
+//! bar takes its place, so dragging and double-click-to-zoom have to be done here).
+//!
+//! Pure policy, no egui widgets: [`command_for`] maps a gesture on the bar's empty space and the
+//! window's current state to the viewport command the host should run.
+
+use egui::ViewportCommand;
+
+/// What the user did on the bar's empty space.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Gesture {
+    /// A primary-button drag began: the window follows the pointer.
+    DragStarted,
+    /// A primary-button double-click: zoom the window, or restore it when already zoomed.
+    DoubleClicked,
+}
+
+/// The window as the host reports it.
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub struct WindowState {
+    /// Zoomed (macOS) / maximized (elsewhere).
+    pub maximized: bool,
+    /// In the OS's full-screen mode.
+    pub fullscreen: bool,
+}
+
+/// The viewport command for `gesture`, if any. Full screen has no title bar to drag or zoom.
+pub fn command_for(gesture: Gesture, window: WindowState) -> Option<ViewportCommand> {
+    if window.fullscreen {
+        return None;
+    }
+    match gesture {
+        Gesture::DragStarted => Some(ViewportCommand::StartDrag),
+        Gesture::DoubleClicked => Some(ViewportCommand::Maximized(!window.maximized)),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const NORMAL: WindowState = WindowState { maximized: false, fullscreen: false };
+    const ZOOMED: WindowState = WindowState { maximized: true, fullscreen: false };
+    const FULL: WindowState = WindowState { maximized: false, fullscreen: true };
+
+    // Scenario: dragging the title bar moves the window
+    #[test]
+    fn given_a_normal_window_when_dragging_then_the_window_follows_the_pointer() {
+        assert_eq!(command_for(Gesture::DragStarted, NORMAL), Some(ViewportCommand::StartDrag));
+    }
+
+    // Scenario: a zoomed window can still be dragged (macOS lets you pull it free)
+    #[test]
+    fn given_a_zoomed_window_when_dragging_then_the_window_follows_the_pointer() {
+        assert_eq!(command_for(Gesture::DragStarted, ZOOMED), Some(ViewportCommand::StartDrag));
+    }
+
+    // Scenario: double-click zooms
+    #[test]
+    fn given_a_normal_window_when_double_clicking_then_it_zooms() {
+        assert_eq!(command_for(Gesture::DoubleClicked, NORMAL), Some(ViewportCommand::Maximized(true)));
+    }
+
+    // Scenario: double-click again restores
+    #[test]
+    fn given_a_zoomed_window_when_double_clicking_then_it_is_restored() {
+        assert_eq!(command_for(Gesture::DoubleClicked, ZOOMED), Some(ViewportCommand::Maximized(false)));
+    }
+
+    // Scenario: full screen ignores both
+    #[test]
+    fn given_a_full_screen_window_then_neither_gesture_does_anything() {
+        assert_eq!(command_for(Gesture::DragStarted, FULL), None);
+        assert_eq!(command_for(Gesture::DoubleClicked, FULL), None);
+    }
+}