fix(gui): close the window on the first click while hosting

Closing the window while a host/viewer child was running took two clicks:
the first only dropped the stream, the second actually closed the window.

eframe drops the app synchronously while it destroys the window, which ran
`ChildProc`'s teardown — SIGINT plus a up-to-2s grace-period wait — on the
event-loop thread. That wait froze the window mid-close, so the first click
looked like it only killed the stream and the window lingered until a second
close event. (The teardown runs from `Drop`, not from an `on_exit` /
`close_requested` hook, so it fires on every backend and close path; those
hooks don't fire at all under some winit backends.)

Make the teardown non-blocking: hold the child in an `Option`, and on drop
SIGINT it synchronously (so the host still runs its ctrl-c teardown even if
we exit immediately after) then reap it on a detached thread instead of
waiting inline. The app drops instantly, so the window closes on the first
click; the kicked-off SIGINT still tears the stream down cleanly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/gui/child.rs

@@ -66,7 +66,9 @@ pub enum CaptureState {
 const STDERR_TAIL_MAX: usize = 60;
 
 pub struct ChildProc {
-    child: Child,
+    /// `Some` while the child is owned here; `Drop` takes it to hand off to a
+    /// detached reaper thread (see the `Drop` impl).
+    child: Option<Child>,
     pub rx: Receiver<ChildEvent>,
     stderr_tail: Arc<Mutex<Vec<String>>>,
     /// Write end of the child's stdin, for the line-based command channel
@@ -123,7 +125,7 @@ impl ChildProc {
         });
 
         Ok(Self {
-            child,
+            child: Some(child),
             rx,
             stderr_tail,
             stdin,
@@ -145,38 +147,42 @@ impl ChildProc {
 
     /// Whether the child is still running.
     pub fn is_alive(&mut self) -> bool {
-        matches!(self.child.try_wait(), Ok(None))
+        matches!(self.child.as_mut().map(Child::try_wait), Some(Ok(None)))
     }
 
     /// The last captured stderr lines, joined — for error display.
     pub fn stderr_tail(&self) -> String {
         self.stderr_tail.lock().unwrap().join("\n")
     }
-
-    /// Gracefully stop the child: SIGINT (so the host runs its ctrl-c teardown
-    /// — tears down capture, closes the endpoint), with a ~2 s grace period
-    /// before a hard kill. Idempotent.
-    pub fn stop(&mut self) {
-        if matches!(self.child.try_wait(), Ok(Some(_))) {
-            return; // already exited
-        }
-        let _ = kill(Pid::from_raw(self.child.id() as i32), Signal::SIGINT);
-        for _ in 0..40 {
-            if matches!(self.child.try_wait(), Ok(Some(_))) {
-                return;
-            }
-            std::thread::sleep(Duration::from_millis(50));
-        }
-        let _ = self.child.kill();
-        let _ = self.child.wait();
-    }
 }
 
 impl Drop for ChildProc {
     fn drop(&mut self) {
-        // Closing the window (dropping the app, hence the session) must not
-        // orphan a live host child streaming to viewers.
-        self.stop();
+        // Leaving a host/viewer screen, or closing the window, must not orphan
+        // a live child — but it must also not *block*. eframe runs this drop
+        // synchronously while it destroys the window, so a grace-period wait
+        // here freezes the window mid-close: the first click looks like it did
+        // nothing (the stream just drops) and the window only goes away on a
+        // second click. So SIGINT now — synchronously, so the host always gets
+        // its ctrl-c teardown (capture down, endpoint closed) even if we exit
+        // right after — then reap on a detached thread instead of waiting.
+        let Some(mut child) = self.child.take() else {
+            return;
+        };
+        if matches!(child.try_wait(), Ok(Some(_))) {
+            return; // already exited; nothing to signal or reap
+        }
+        let _ = kill(Pid::from_raw(child.id() as i32), Signal::SIGINT);
+        std::thread::spawn(move || {
+            for _ in 0..40 {
+                if matches!(child.try_wait(), Ok(Some(_))) {
+                    return;
+                }
+                std::thread::sleep(Duration::from_millis(50));
+            }
+            let _ = child.kill();
+            let _ = child.wait();
+        });
     }
 }