Clear composer on fork (#3445)

Fixes this

<img width="344" height="51" alt="image"
src="https://github.com/user-attachments/assets/f227d338-b044-4f8d-bf07-87499b4230d8"
/>

codex-rs/tui/src/app_backtrack.rs

@@ -335,7 +335,7 @@ impl App {
         self.trim_transcript_for_backtrack(drop_count);
         self.render_transcript_once(tui);
         if !prefill.is_empty() {
-            self.chat_widget.insert_str(prefill);
+            self.chat_widget.set_composer_text(prefill.to_string());
         }
         tui.frame_requester().schedule_frame();
     }

codex-rs/tui/src/bottom_pane/chat_composer.rs

@@ -243,6 +243,10 @@ impl ChatComposer {
 
     /// Replace the entire composer content with `text` and reset cursor.
     pub(crate) fn set_text_content(&mut self, text: String) {
+        // Clear any existing content, placeholders, and attachments first.
+        self.textarea.set_text("");
+        self.pending_pastes.clear();
+        self.attached_images.clear();
         self.textarea.set_text(&text);
         self.textarea.set_cursor(0);
         self.sync_command_popup();

codex-rs/tui/src/chatwidget.rs

@@ -1316,6 +1316,11 @@ impl ChatWidget {
         self.bottom_pane.insert_str(text);
     }
 
+    /// Replace the composer content with the provided text and reset cursor.
+    pub(crate) fn set_composer_text(&mut self, text: String) {
+        self.bottom_pane.set_composer_text(text);
+    }
+
     pub(crate) fn show_esc_backtrack_hint(&mut self) {
         self.bottom_pane.show_esc_backtrack_hint();
     }