set_window_title() doesn't flush, so a title set during a command appears late #1782

Problem

Cmd.set_window_title() calls prompt-toolkit's set_title(), which only writes the escape sequence into the buffer of the app session's output (Vt100_Output.write_raw()). Nothing flushes that buffer until something else does, such as the next prompt render or an async alert being printed. So a title set from inside a command or a hook doesn't change until the command finishes.

For example, a command which calls self.set_window_title("working...") and then runs for 5 seconds only changes the title after those 5 seconds. The textual_tty_embed.py example in #1781 works around this by calling get_app_session().output.flush() itself after setting the title.

Why not just call flush() in set_window_title()

From reading the code:

• In the default case, where self.stdout is sys.stdout, _create_prompt_session() doesn't pass an output to PromptSession, so the prompt draws through the same Vt100_Output that set_title() writes into.
• set_window_title() is documented next to the async alert functions, and examples/async_printing.py calls it from a background thread while the prompt is active.
• Vt100_Output.flush() swaps an unlocked list (data = "".join(self._buffer); self._buffer = []). Flushing from a background thread while the renderer flushes on the main thread can drop or duplicate output. Arguably the existing write_raw() from a background thread is already racy in the same way.
• The command toolbar keeps a prompt-toolkit application alive while a command runs, so "no prompt is drawing" can't be assumed during a command either.

Possible approach

When a prompt-toolkit application is running, schedule the title change and a flush on its event loop with loop.call_soon_threadsafe(). Otherwise, write the title and flush directly. This needs tests for both paths, a check of the command toolbar and pager code paths, and a look at Windows, where Win32Output.set_title() uses SetConsoleTitleW.

Once this is fixed, the workaround in examples/textual_tty_embed.py and the "prompt-toolkit buffers its output" bullet in docs/examples/embedding_in_textual.md can be removed.