add theme titlebar height and app-owned strip dragging

crates/layout/src/window_controls.rs

@@ -1,17 +1,17 @@
 //! The window controls: minimize, maximize/restore, close — in the platform's
-//! own dress and behavior. The titlebar strips drag the window themselves via
-//! `WindowControlArea::Drag`; macOS draws its traffic lights in the shell's
-//! reserved pad and needs no controls of ours.
+//! own dress and behavior. Titlebar strips are app-owned: they move the
+//! window themselves via [`titlebar_strip`], so a dragged tab or button press
+//! never moves it; macOS draws its traffic lights in the shell's reserved pad
+//! and needs no controls of ours.
 
-use gpui::{App, ClickEvent, ElementId, IntoElement, RenderOnce, Window, div};
+use gpui::WindowControlArea;
+use gpui::{App, ClickEvent, Div, ElementId, IntoElement, RenderOnce, Stateful, Window, div};
+use gpui::{InteractiveElement as _, MouseButton, StatefulInteractiveElement as _};
 
 #[cfg(any(target_os = "windows", target_os = "linux"))]
 use gpui::prelude::FluentBuilder as _;
 #[cfg(any(target_os = "windows", target_os = "linux"))]
-use gpui::{
-    AnyElement, InteractiveElement as _, ParentElement as _, StatefulInteractiveElement as _,
-    Styled as _,
-};
+use gpui::{AnyElement, ParentElement as _};
 #[cfg(any(target_os = "windows", target_os = "linux"))]
 use gpui_native_icons::{IconRole, icon};
 #[cfg(any(target_os = "windows", target_os = "linux"))]
@@ -20,7 +20,7 @@ use gpui_native_theme::ActiveTheme as _;
 #[cfg(target_os = "linux")]
 use gpui::Decorations;
 #[cfg(target_os = "windows")]
-use gpui::{WindowControlArea, px, rgba};
+use gpui::rgba;
 
 /// macOS traffic-lights zone: the pad a leading titlebar strip reserves.
 pub const TRAFFIC_LIGHT_PAD: f32 = 78.0;
@@ -164,6 +164,40 @@ pub fn maximize_on_double_click() -> impl Fn(&ClickEvent, &mut Window, &mut App)
     }
 }
 
+/// Makes an element a titlebar strip: pressing and dragging it moves the
+/// window, double-clicking maximizes, and Windows keeps its native caption
+/// behavior through the `Drag` control area.
+pub fn titlebar_strip(this: Stateful<Div>, window: &mut Window, cx: &mut App) -> Stateful<Div> {
+    let should_move = window.use_state(cx, |_, _| false);
+    this.window_control_area(WindowControlArea::Drag)
+        .on_mouse_down_out(window.listener_for(&should_move, |should_move, _, _, _| {
+            *should_move = false;
+        }))
+        .on_mouse_down(
+            MouseButton::Left,
+            window.listener_for(&should_move, |should_move, _, _, _| {
+                *should_move = true;
+            }),
+        )
+        .on_mouse_up(
+            MouseButton::Left,
+            window.listener_for(&should_move, |should_move, _, _, _| {
+                *should_move = false;
+            }),
+        )
+        .on_mouse_move(
+            window.listener_for(&should_move, |should_move, _, window, cx| {
+                if *should_move {
+                    *should_move = false;
+                    if !cx.has_active_drag() {
+                        window.start_window_move();
+                    }
+                }
+            }),
+        )
+        .on_click(maximize_on_double_click())
+}
+
 /// One Windows caption button.
 #[cfg(target_os = "windows")]
 fn caption_button(