gpui: Move 'app closing on last window closed' behavior to app-side (#41436)

This commit is a continuation of #36548. As per [mikayla-maki's
Comment](https://github.com/zed-industries/zed/pull/36548#issuecomment-3412140698),
I removed the process management behavior located in GPUI and
reimplemented it in Zed.

Release Notes:

- N/A

---------

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/gpui/src/platform/linux/wayland/client.rs

@@ -387,9 +387,6 @@ impl WaylandClientStatePtr {
         {
             state.keyboard_focused_window = Some(window);
         }
-        if state.windows.is_empty() {
-            state.common.signal.stop();
-        }
     }
 }
 

crates/gpui/src/platform/linux/x11/client.rs

@@ -246,10 +246,6 @@ impl X11ClientStatePtr {
             state.keyboard_focused_window = None;
         }
         state.cursor_styles.remove(&x_window);
-
-        if state.windows.is_empty() {
-            state.common.signal.stop();
-        }
     }
 
     pub fn update_ime_position(&self, bounds: Bounds<Pixels>) {

crates/gpui/src/platform/windows/platform.rs

@@ -753,9 +753,7 @@ impl WindowsPlatformInner {
         }
         match message {
             WM_GPUI_CLOSE_ONE_WINDOW => {
-                if self.close_one_window(HWND(lparam.0 as _)) {
-                    unsafe { PostQuitMessage(0) };
-                }
+                self.close_one_window(HWND(lparam.0 as _));
                 Some(0)
             }
             WM_GPUI_TASK_DISPATCHED_ON_MAIN_THREAD => self.run_foreground_task(),

crates/zed/src/zed.rs

@@ -274,16 +274,27 @@ pub fn init(cx: &mut App) {
 }
 
 fn bind_on_window_closed(cx: &mut App) -> Option<gpui::Subscription> {
-    WorkspaceSettings::get_global(cx)
-        .on_last_window_closed
-        .is_quit_app()
-        .then(|| {
-            cx.on_window_closed(|cx| {
-                if cx.windows().is_empty() {
-                    cx.quit();
-                }
+    #[cfg(target_os = "macos")]
+    {
+        WorkspaceSettings::get_global(cx)
+            .on_last_window_closed
+            .is_quit_app()
+            .then(|| {
+                cx.on_window_closed(|cx| {
+                    if cx.windows().is_empty() {
+                        cx.quit();
+                    }
+                })
             })
-        })
+    }
+    #[cfg(not(target_os = "macos"))]
+    {
+        Some(cx.on_window_closed(|cx| {
+            if cx.windows().is_empty() {
+                cx.quit();
+            }
+        }))
+    }
 }
 
 pub fn build_window_options(display_uuid: Option<Uuid>, cx: &mut App) -> WindowOptions {