Theme · Platform: a title bar in three parts, buttons at the window's rem

A title bar lays out a start, a title and an end. In the centered styles
the start and end grow alike from nothing, their padding inside them, so
a short title sits at the bar's center and a long one gives way before
whatever stands at either end; on Windows the title, or a spacer when
there is none, takes the free width before the buttons. Caption buttons
take the bar's height at the window's own rem. Tests keep an untitled
Windows bar's buttons at its end, a long Linux title before the round
buttons, a short title centered, and the buttons inside a bar at rem 8.

src/shell/tests.rs

@@ -190,3 +190,74 @@ fn caption_buttons_keep_their_height_in_a_wrapping_row(cx: &mut TestAppContext)
     assert!(combo.top() > row.top(), "the caps wrap below the buttons");
     assert!(combo.bottom() <= row.bottom(), "{combo:?} inside {row:?}");
 }
+
+struct Bar(Option<&'static str>, crate::theme::Platform);
+
+impl Render for Bar {
+    fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
+        let bar = super::TitleBar::new("bar").platform(self.1);
+        div()
+            .w(px(280.0))
+            .flex()
+            .flex_col()
+            .child(
+                div()
+                    .debug_selector(|| "bar-box".into())
+                    .child(match self.0 {
+                        Some(title) => bar.title(title),
+                        None => bar,
+                    }),
+            )
+    }
+}
+
+/// A Windows bar with no title still keeps its buttons at its right end.
+#[gpui::test]
+fn an_untitled_windows_bar_keeps_its_buttons_at_the_end(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let (_, cx) = cx.add_window_view(|_, _| Bar(None, crate::theme::Platform::Windows));
+    settle(cx);
+    let close = cx.debug_bounds("caption-Close").expect("a close button");
+    assert_eq!(close.right(), px(280.0));
+}
+
+/// A long centered title ends before the Linux round buttons.
+#[gpui::test]
+fn a_long_centered_title_ends_before_the_buttons(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let long = "Ely — Projects at a narrow width, and more besides";
+    let (_, cx) = cx.add_window_view(|_, _| Bar(Some(long), crate::theme::Platform::Linux));
+    settle(cx);
+    let title = cx.debug_bounds("titlebar-title").expect("the title");
+    let controls = cx.debug_bounds("titlebar-controls").expect("the buttons");
+    assert!(
+        title.right() <= controls.left(),
+        "{title:?} before {controls:?}"
+    );
+}
+
+/// At half the rem size the caption buttons shrink with the bar and stay inside it.
+#[gpui::test]
+fn caption_buttons_follow_the_windows_rem(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let (_, cx) = cx.add_window_view(|_, _| Bar(None, crate::theme::Platform::Windows));
+    cx.update(|window, _| window.set_rem_size(px(8.0)));
+    settle(cx);
+    let bar = cx.debug_bounds("bar-box").expect("the bar");
+    let close = cx.debug_bounds("caption-Close").expect("a close button");
+    assert!(
+        close.top() >= bar.top() && close.bottom() <= bar.bottom(),
+        "{close:?} inside {bar:?}"
+    );
+}
+
+/// A short title sits at the bar's center, whatever stands at either end.
+#[gpui::test]
+fn a_short_title_sits_at_the_center(cx: &mut TestAppContext) {
+    cx.update(Theme::init);
+    let (_, cx) = cx.add_window_view(|_, _| Bar(Some("Notes"), crate::theme::Platform::Linux));
+    settle(cx);
+    let title = cx.debug_bounds("titlebar-title").expect("the title");
+    let middle = title.left() + title.size.width / 2.0;
+    assert!((middle - px(140.0)).abs() < px(1.0), "{title:?} centered");
+}