Move command spawning to a thread

It was showing up on profiles causing dropped frames.

src/input.rs

@@ -118,9 +118,7 @@ impl State {
                         self.backend.toggle_debug_tint();
                     }
                     Action::Spawn(command) => {
-                        if let Some((command, args)) = command.split_first() {
-                            spawn(command, args);
-                        }
+                        spawn(command);
                     }
                     Action::ScreenshotScreen => {
                         let active = self.niri.layout.active_output().cloned();

src/main.rs

@@ -164,14 +164,10 @@ fn main() {
     };
 
     // Spawn commands from cli and auto-start.
-    if let Some((command, args)) = cli.command.split_first() {
-        spawn(command, args);
-    }
+    spawn(cli.command);
 
     for elem in spawn_at_startup {
-        if let Some((command, args)) = elem.command.split_first() {
-            spawn(command, args);
-        }
+        spawn(elem.command);
     }
 
     // Run the compositor.

src/utils.rs

@@ -6,6 +6,7 @@ use std::path::PathBuf;
 use std::process::{Command, Stdio};
 use std::ptr::null_mut;
 use std::sync::atomic::{AtomicBool, Ordering};
+use std::thread;
 use std::time::Duration;
 
 use anyhow::{ensure, Context};
@@ -58,7 +59,27 @@ pub static REMOVE_ENV_RUST_BACKTRACE: AtomicBool = AtomicBool::new(false);
 pub static REMOVE_ENV_RUST_LIB_BACKTRACE: AtomicBool = AtomicBool::new(false);
 
 /// Spawns the command to run independently of the compositor.
-pub fn spawn(command: impl AsRef<OsStr>, args: impl IntoIterator<Item = impl AsRef<OsStr>>) {
+pub fn spawn<T: AsRef<OsStr> + Send + 'static>(command: Vec<T>) {
+    let _span = tracy_client::span!();
+
+    if command.is_empty() {
+        return;
+    }
+
+    // Spawning and waiting takes some milliseconds, so do it in a thread.
+    let res = thread::Builder::new()
+        .name("Command Spawner".to_owned())
+        .spawn(move || {
+            let (command, args) = command.split_first().unwrap();
+            spawn_sync(command, args);
+        });
+
+    if let Err(err) = res {
+        warn!("error spawning a thread to spawn the command: {err:?}");
+    }
+}
+
+fn spawn_sync(command: impl AsRef<OsStr>, args: impl IntoIterator<Item = impl AsRef<OsStr>>) {
     let _span = tracy_client::span!();
 
     let command = command.as_ref();