fix(agents): type a fork or launch command at the new shell's first prompt

Sent the moment the pane was up, the line was echoed by the tty above
everything the shell printed while starting, then read after it. It now
waits for the first prompt, for at most three seconds.

src/ui/agent_launch.rs

@@ -147,11 +147,38 @@ pub(crate) fn fork_line(
     agent.fork_command(session_id, Some(&argv))
 }
 
+/// How long a new shell is given to reach its first prompt before a queued
+/// command is typed anyway.
+const PROMPT_WAIT: std::time::Duration = std::time::Duration::from_secs(3);
+const PROMPT_POLL: std::time::Duration = std::time::Duration::from_millis(50);
+
 /// Type `command` into the shell `slot` holds — now if it is up, or the
 /// moment it lands if it is still connecting.
+///
+/// A shell that is up is not yet at its prompt: typed straight in, the line
+/// was echoed by the tty above everything the shell prints while it starts (a
+/// banner, `fastfetch`) and read only afterwards. It waits for the shell's
+/// first prompt instead — up to [`PROMPT_WAIT`], for a shell with no
+/// integration to say when that is.
 pub(crate) fn run_when_ready(slot: &PaneSlot, command: String, cx: &mut App) {
     match slot {
-        PaneSlot::Ready(view) => view.read(cx).run_command_line(&command),
+        PaneSlot::Ready(view) => {
+            let view = view.downgrade();
+            cx.spawn(async move |cx| {
+                let deadline = std::time::Instant::now() + PROMPT_WAIT;
+                loop {
+                    let ready = view
+                        .read_with(cx, |view, _| view.terminal.at_prompt())
+                        .unwrap_or(true);
+                    if ready || std::time::Instant::now() >= deadline {
+                        break;
+                    }
+                    cx.background_executor().timer(PROMPT_POLL).await;
+                }
+                let _ = view.read_with(cx, |view, _| view.run_command_line(&command));
+            })
+            .detach();
+        }
         PaneSlot::Connecting(pending) => {
             pending.update(cx, |pending, _| pending.spawn.run_on_land = Some(command));
         }