feat: add a close button to dialog

The close affordance reuses Button (ghost, small) at the panel's top
right and routes through on_change like escape and scrim clicks; it is
hidden in alert mode. The dialog feature now depends on button, and
AGENTS.md codifies preferring existing components over custom elements.

AGENTS.md

@@ -1,5 +1,6 @@
 # Coding guidelines
 
+- Prefer existing components (e.g. build a close affordance out of `Button`) over custom elements; build a custom element only when no existing component fits.
 - Prioritize code correctness and clarity. Speed and efficiency are secondary priorities unless otherwise specified.
 - Do not write organizational or comments that summarize the code. Comments should only be written to explain "why" the code is written in some way in the case there is a reason that is tricky / non-obvious.
 - Keep doc comments and comments simple, one line, straightforward, and unbloated.

crates/core/Cargo.toml

@@ -76,7 +76,7 @@ tooltip = []
 menu = ["scroll_area"]
 dropdown_menu = ["menu", "button"]
 select = ["menu"]
-dialog = []
+dialog = ["button"]
 empty_state = []
 alert = []
 toast = []

crates/core/src/dialog.rs

@@ -6,14 +6,17 @@ use gpui::{
     IntoElement, KeyDownEvent, MouseButton, ParentElement as _, RenderOnce, Role, SharedString,
     StatefulInteractiveElement as _, Styled as _, Window, canvas, div, ease_out_quint, relative,
 };
+use gpui_native_icons::IconRole;
 use gpui_native_theme::ActiveTheme as _;
 
-use crate::common::v_flex;
+use crate::button::{Button, ButtonVariant};
+use crate::common::{Sizable, v_flex};
 
 /// Requests the dialog's close: `false` closes.
 pub type ChangeHandler = Rc<dyn Fn(bool, &mut Window, &mut App)>;
 type ContentFn = Box<dyn Fn(&mut Window, &mut App) -> AnyElement>;
 type ActionsFn = Box<dyn Fn(&mut Window, &mut App) -> Vec<AnyElement>>;
+
 /// A centred panel over a dim scrim: the platform's modal.
 #[derive(IntoElement)]
 pub struct Dialog {
@@ -57,7 +60,7 @@ impl Dialog {
         self
     }
 
-    /// Requests close through `on_change` on escape and scrim clicks.
+    /// Requests close through `on_change` on escape, scrim clicks, and the close button.
     pub fn on_change<F>(mut self, handler: F) -> Self
     where
         F: Fn(bool, &mut Window, &mut App) + 'static,
@@ -154,6 +157,7 @@ impl RenderOnce for Dialog {
 
         let on_change = self.on_change.clone();
         let alert = self.alert;
+        let show_close = !alert && on_change.is_some();
 
         // Keyboard focus rides the panel across frames.
         let focus = gpui_native_utils::focus_handle(&self.id, window, cx);
@@ -190,12 +194,40 @@ impl RenderOnce for Dialog {
                     cx.stop_propagation();
                 }
             })
-            .when_some(self.title, |panel, title| {
+            .when(self.title.is_some() || show_close, |panel| {
                 panel.child(
                     div()
-                        .text_size(title_size)
-                        .font_weight(FontWeight::SEMIBOLD)
-                        .child(title),
+                        .flex()
+                        .items_center()
+                        .justify_end()
+                        .gap_2()
+                        .when_some(self.title.clone(), |row, title| {
+                            row.child(
+                                div()
+                                    .flex_1()
+                                    .text_size(title_size)
+                                    .font_weight(FontWeight::SEMIBOLD)
+                                    .child(title),
+                            )
+                        })
+                        .when(show_close, |row| {
+                            row.child(
+                                Button::new("dialog-close")
+                                    .icon(IconRole::Close)
+                                    .variant(ButtonVariant::Ghost)
+                                    .small()
+                                    .debug_selector(|| "dialog-close".to_string())
+                                    .accessibility_label("Close")
+                                    .on_click({
+                                        let on_change = on_change.clone();
+                                        move |_, window, cx| {
+                                            if let Some(on_change) = &on_change {
+                                                on_change(false, window, cx);
+                                            }
+                                        }
+                                    }),
+                            )
+                        }),
                 )
             })
             .when_some(content, |panel, content| {

crates/core/tests/dialog.rs

@@ -158,3 +158,26 @@ fn clicking_the_panel_does_not_dismiss() {
     click(&mut visual, point(px(960.), px(540.)));
     assert!(changes.borrow().is_empty());
 }
+
+/// Clicking the close button reports close like escape and the scrim; the
+/// panel then unmounts the button with the rest of the dialog.
+#[test]
+fn the_close_button_reports_close_and_disappears() {
+    let (_cx, mut visual, changes) = dialog_window(true, false);
+
+    let bounds = visual
+        .debug_bounds("dialog-close")
+        .expect("the close button rendered");
+    click(&mut visual, bounds.center());
+    assert_eq!(*changes.borrow(), vec![false]);
+
+    draw(&mut visual);
+    assert!(visual.debug_bounds("dialog-close").is_none());
+}
+
+/// An alert answers only through its actions, so it renders no close button.
+#[test]
+fn an_alert_hides_the_close_button() {
+    let (_cx, mut visual, _changes) = dialog_window(true, true);
+    assert!(visual.debug_bounds("dialog-close").is_none());
+}