Toga 0.5.3: Using MultilineTextInput with GTK backend causes hang

### Describe the bug

On Kubuntu 25.10 (KDE Desktop)
After adding new requirements I wanted to to perform `briefcase update -r`. Then apparently, the toga was also updated to 0.5.3.

I was always using `briefcase dev` or directly `python -m app` and did not notice until `briefcase run`

Then my UI window did not appear. Added trace for each line and everything was visible until this line:

```
self.logs = toga.MultilineTextInput(
readonly=True,
style=Pack(font_family="Segoe UI Emoji")
)
```

Then, the app window did not appear.

Pinning toga-gtk and toga version on `[tool.briefcase.app.lf.linux]` to 0.5.2 fixed the issue temporarily and my program worked as before.

### Steps to reproduce

1. Perform `briefcase update -r`
2. Make sure you have toga==0.5.3
3. Use multiline text input
4. Continue creating main_container
5. Run the app via briefcase update && briefcase build && briefcase run
6. (It is also possible to run it via python -m app)

### Expected behavior

It should work as in 0.5.2

### Screenshots

_No response_

### Environment

- Operating System: Kubuntu 25.10
- Python version: 3.13.7
- Software versions:
- Briefcase: 0.3.26
- Toga: 0.5.3

### Logs

### Additional context

Linux section in project.toml was untouched.

```
[tool.briefcase.app.lf.linux]
requires = [
"toga-gtk~=0.5.0",
# PyGObject 3.52.1 enforces a requirement on libgirepository-2.0-dev. This library
# isn't available on Debian 12/Ubuntu 22.04. If you don't need to support those (or
# older) releases, you can remove this version pin. See beeware/toga#3143.
"pygobject < 3.52.1",
]
```

May be related to https://github.com/beeware/toga/issues/1142, #3914 or https://github.com/beeware/toga/issues/3658.

Reproduce the hang with Toga 0.5.3 using a read-only MultilineTextInput and the GTK backend, first with `python -m app` and then with `briefcase update && briefcase build && briefcase run`. Compare against Toga 0.5.2 and review the related issues #1142, #3914, and #3658. Done means the window opens and the multiline input works as it did in 0.5.2.

Written by the indexing model from the issue text.