TitleBar: a press its buttons claim never moves the window

The drag area armed a window move on every press, buttons included, so a
click on Linux's drawn buttons that moved a pixel started a move and the
compositor kept the release. It now arms only on unclaimed presses.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/shell/chrome.rs

@@ -21,8 +21,11 @@ pub fn drag_region(id: impl Into<ElementId>, window: &mut Window, cx: &mut App)
     div()
         .id(id)
         .window_control_area(WindowControlArea::Drag)
-        .on_mouse_down(MouseButton::Left, move |_, _, cx| {
+        .on_mouse_down(MouseButton::Left, move |_, window, cx| {
-            down.update(cx, |pressed, _| *pressed = true)
+            // A button inside claims its own press.
+            if !window.default_prevented() {
+                down.update(cx, |pressed, _| *pressed = true)
+            }
         })
         .on_mouse_move(move |_, window, cx| {
             if *moved.read(cx) {

src/shell/mod.rs

@@ -11,6 +11,8 @@ mod switcher;
 mod tabs;
 #[cfg(all(test, feature = "test-support"))]
 mod tests;
+#[cfg(all(test, feature = "test-support"))]
+mod title_tests;
 mod update;
 mod vibrancy;
 mod windows;

src/shell/title_tests.rs

@@ -0,0 +1,73 @@
+use gpui::{
+    Context, IntoElement, Modifiers, MouseButton, MouseDownEvent, MouseMoveEvent, MouseUpEvent,
+    ParentElement, Point, Render, TestAppContext, Window, div, point, prelude::*, px,
+};
+
+use super::TitleBar;
+use crate::theme::{Platform, Theme};
+
+struct Bar {
+    closes: usize,
+}
+
+impl Render for Bar {
+    fn render(&mut self, _: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        let view = cx.entity();
+        div().w(px(600.0)).child(
+            TitleBar::new("bar")
+                .platform(Platform::Linux)
+                .on_close(move |_, cx| view.update(cx, |bar, _| bar.closes += 1)),
+        )
+    }
+}
+
+/// Presses at `at`, moves two pixels, releases.
+fn wobble(at: Point<gpui::Pixels>, cx: &mut gpui::VisualTestContext) {
+    let moved = at + point(px(2.0), px(1.0));
+    cx.simulate_event(MouseDownEvent {
+        button: MouseButton::Left,
+        position: at,
+        modifiers: Modifiers::none(),
+        click_count: 1,
+        first_mouse: false,
+    });
+    cx.simulate_event(MouseMoveEvent {
+        position: moved,
+        pressed_button: Some(MouseButton::Left),
+        modifiers: Modifiers::none(),
+    });
+    cx.simulate_event(MouseUpEvent {
+        button: MouseButton::Left,
+        position: moved,
+        modifiers: Modifiers::none(),
+        click_count: 1,
+    });
+    cx.run_until_parked();
+}
+
+/// A press that wobbles on Linux's close button still closes; it never moves the window.
+#[gpui::test]
+fn a_wobbling_press_on_a_linux_button_clicks_it(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let (view, cx) = cx.add_window_view(|_, _| Bar { closes: 0 });
+    cx.run_until_parked();
+    let controls = cx
+        .debug_bounds("titlebar-controls")
+        .expect("drawn controls");
+    // Close is the last of the round buttons.
+    wobble(point(controls.right() - px(8.0), controls.center().y), cx);
+    assert_eq!(view.read_with(cx, |bar, _| bar.closes), 1);
+}
+
+/// The bar's empty space still moves the window (the test window panics on a move).
+#[gpui::test]
+#[should_panic(expected = "not implemented")]
+fn a_wobbling_press_on_the_bar_moves_the_window(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let (_, cx) = cx.add_window_view(|_, _| Bar { closes: 0 });
+    cx.run_until_parked();
+    let controls = cx
+        .debug_bounds("titlebar-controls")
+        .expect("drawn controls");
+    wobble(point(controls.left() - px(200.0), controls.center().y), cx);
+}