...

kitty_tests/__init__.py

@@ -255,10 +255,12 @@ class PTY:
                 del self.slave_fd
             del self.master_fd
 
-    def write_to_child(self, data):
+    def write_to_child(self, data, flush=False):
         if isinstance(data, str):
             data = data.encode('utf-8')
         self.write_buf += data
+        if flush:
+            self.process_input_from_child(0)
 
     def send_cmd_to_child(self, cmd):
         self.write_to_child(cmd + '\r')

kitty_tests/prewarm.py

@@ -93,7 +93,7 @@ def socket_child_main(exit_code=0, initial_print=''):
         pty.wait_till(lambda: 'child ready:' in pty.screen_contents())
         pty.set_window_size(columns=cols + 3)
         pty.wait_till(lambda: f'Screen size changed: {cols + 3}' in pty.screen_contents())
-        pty.write_to_child('\x03' * 64)
+        pty.write_to_child('\x03' * 64, flush=True)
         wait_for_death(signal.SIGINT, timeout=30)
         pty.wait_till(lambda: 'KeyboardInterrupt' in pty.screen_contents(), timeout=30)