Setting a recomposing reactive attribute after push_screen() causes Header widget crash

[cdxjcl]

In a ModalScreen subclass, I defined a reactive attribute with recompose=True. If I set this attribute after calling App.push_screen(screen), the Header widget crashes unexpectedly. I'm not sure whether this is a bug or a misuse on my part. Would appreciate any guidance or clarification.

This is the traceback:

This is test code：

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.events import Click
from textual.widgets import (
    Footer,
    Header,
    Placeholder,
)
from textual.screen import ModalScreen


class BarScreen(ModalScreen):
    BINDINGS = [("escape", "app.pop_screen", "Cancel")]

    bar: reactive[str] = reactive("init", recompose=True)

    def compose(self) -> ComposeResult:
        yield Header()
        yield Placeholder("bar")
        yield Footer()

class FooApp(App):
    BINDINGS = [
        ("q", "quit", "Quit"),
    ]

    def compose(self) -> ComposeResult:
        yield Header()
        yield Placeholder("foo")
        yield Footer()

    def on_click(self, event: Click) -> None:
        screen = BarScreen()
        self.push_screen(screen)
        screen.bar = "foo"


if __name__ == "__main__":
    app = FooApp()
    app.run()

[willmcgugan]

I can't think of a good reason to recompose an entire screen. You might want to elaborate on why you wanted to do that.

That said, I suspect the issue is that you aren't awaiting push_screen. If you add a widget or a screen to the UI it doesn't complete immediately. Textual only guarantees it will be complete before the next event is handled.

The fix is to make the handler async, and await push_screen so that it is finished by the following line:

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.events import Click
from textual.widgets import (
    Footer,
    Header,
    Placeholder,
)
from textual.screen import ModalScreen


class BarScreen(ModalScreen):
    BINDINGS = [("escape", "app.pop_screen", "Cancel")]

    bar: reactive[str] = reactive("init", recompose=True)

    def compose(self) -> ComposeResult:
        yield Header()
        yield Placeholder("bar")
        yield Footer()

class FooApp(App):
    BINDINGS = [
        ("q", "quit", "Quit"),
    ]

    def compose(self) -> ComposeResult:
        yield Header()
        yield Placeholder("foo")
        yield Footer()

    async def on_click(self, event: Click) -> None:
        screen = BarScreen()
        await self.push_screen(screen)
        screen.bar = "foo"


if __name__ == "__main__":
    app = FooApp()
    app.run()
    ```

[cdxjcl]

I was trying to display an image inside a Screen using a custom widget that wraps the Image widget from textual-image. I wanted to bind a url reactive attribute from the Screen to the one inside the custom widget using data_bind.

Since I was referencing the textual-image demo, I had copied the recompose=True setting from the demo without realizing it might cause issues.

Also, even when using the async handler with await push_screen() as suggested, the crash still occurs.

message_pump.py:726: RuntimeWarning: coroutine 'Header._on_mount.<locals>.set_title' was never awaited
  await self._flush_next_callbacks()
RuntimeWarning: Enable tracemalloc to get the object allocation traceback

[willmcgugan]

The code above works for me. Make sure you are running the latest Textual.

[cdxjcl]

I'm using the latest version of Textual, running on Windows 11 with Windows Terminal. The crash also occurs when testing inside a WSL Linux environment, so it feels like some kind of race condition. Below is the output from textual diagnose:

Textual Diagnostics

Versions

Name	Value
Textual	6.1.0
Rich	14.1.0

Python

Name	Value
Version	3.13.5
Implementation	CPython
Compiler	MSC v.1943 64 bit (AMD64)
Executable	C:\Users\xx.venv\Scripts\python.exe

Operating System

Name	Value
System	Windows
Release	11
Version	10.0.26100

Terminal

Name	Value
Terminal Application	Windows Terminal
TERM	Not set
COLORTERM	Not set
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=165, height=48
legacy_windows	False
min_width	1
max_width	165
is_terminal	True
encoding	utf-8
max_height	48
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None