host: route graceful closes of helper windows to the app window (Windows)

tao's thread event target and the single-instance message window are visible
1 px unowned top-level windows. taskkill /PID (WM_CLOSE, no /F) closes only the
first such window in z-order; a minimized app window sinks below them, so the
close destroyed a helper and the app kept running. route_helper_closes
subclasses those helpers and forwards WM_CLOSE to the app window.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/waybetter-host/src/host.rs

@@ -185,6 +185,77 @@ pub fn system_menu<R: Runtime>(webview: &Webview<R>, x: f64, y: f64) -> Result<(
     Ok(())
 }
 
+/// Windows: a graceful close sent to one of the process's helper windows closes the app window
+/// instead. Call once from `setup`, on the main thread, after the main window is built. No-op
+/// elsewhere.
+///
+/// tao's thread event target and the single-instance plugin's message window are visible (1 px,
+/// layered, transparent) unowned top-level windows. `taskkill /PID` without `/F` posts one
+/// `WM_CLOSE` to the first such window in z-order; a minimized window sinks to the bottom, so the
+/// close hit a helper, `DefWindowProc` destroyed it (a second instance could no longer find us,
+/// or the event loop lost its target) and the app kept running.
+pub fn route_helper_closes<R: Runtime>(window: &tauri::Window<R>) {
+    #[cfg(windows)]
+    helper_close::install(window);
+    #[cfg(not(windows))]
+    let _ = window;
+}
+
+#[cfg(windows)]
+mod helper_close {
+    use windows::core::BOOL;
+    use windows::Win32::Foundation::{HWND, LPARAM, LRESULT, WPARAM};
+    use windows::Win32::System::Threading::GetCurrentThreadId;
+    use windows::Win32::UI::Shell::{DefSubclassProc, RemoveWindowSubclass, SetWindowSubclass};
+    use windows::Win32::UI::WindowsAndMessaging::{
+        EnumThreadWindows, GetWindow, IsWindow, IsWindowVisible, PostMessageW, GW_OWNER, WM_CLOSE, WM_NCDESTROY,
+    };
+
+    const SUBCLASS_ID: usize = 0x7762_636c; // "wbcl"
+
+    pub(super) fn install<R: tauri::Runtime>(window: &tauri::Window<R>) {
+        let Ok(main) = window.hwnd() else { return };
+        // SAFETY: enumerates and subclasses windows of the calling (event loop) thread only;
+        // `main` is passed by value through the LPARAM / ref data.
+        unsafe {
+            let _ = EnumThreadWindows(GetCurrentThreadId(), Some(visit), LPARAM(main.0 as isize));
+        }
+    }
+
+    unsafe extern "system" fn visit(hwnd: HWND, main: LPARAM) -> BOOL {
+        let main = HWND(main.0 as *mut _);
+        // Visible, unowned, not the app window: what `taskkill` may pick. IME windows are hidden.
+        if hwnd != main && IsWindowVisible(hwnd).as_bool() && GetWindow(hwnd, GW_OWNER).is_err() {
+            let _ = SetWindowSubclass(hwnd, Some(forward), SUBCLASS_ID, main.0 as usize);
+        }
+        BOOL(1)
+    }
+
+    unsafe extern "system" fn forward(
+        hwnd: HWND,
+        msg: u32,
+        wparam: WPARAM,
+        lparam: LPARAM,
+        id: usize,
+        main: usize,
+    ) -> LRESULT {
+        match msg {
+            WM_CLOSE => {
+                let main = HWND(main as *mut _);
+                if IsWindow(Some(main)).as_bool() {
+                    let _ = PostMessageW(Some(main), WM_CLOSE, WPARAM(0), LPARAM(0));
+                    return LRESULT(0);
+                }
+            }
+            WM_NCDESTROY => {
+                let _ = RemoveWindowSubclass(hwnd, Some(forward), id);
+            }
+            _ => {}
+        }
+        DefSubclassProc(hwnd, msg, wparam, lparam)
+    }
+}
+
 /// Places a child webview (tab, popover) at a rectangle of its window, in logical px.
 ///
 /// Windows/macOS: the native child bounds. Linux: tauri 2 packs every child webview into the