Widgets within an OptionContainer are too large if a window is shown prior to the container being assigned to the window

### Describe the bug

On Windows, a behavior that was introduced in Toga 0.5.0 in which (at least) Label and Button elements within an OptionContainer are too large if the OptionContainer is assigned to a Window after the Window has already been shown.

Here is code to reproduce:

```
import toga

class TogaApp(toga.App):
def startup(self) -> None:
self.main_window = toga.MainWindow()
self.main_window.show()

container_box = toga.Box()
container_box.add(toga.Label("TEST Label", background_color="lightblue"))
container_box.add(toga.Button("TEST Button"))
home_tab = toga.OptionItem(text="Home", content=container_box,)
option_container = toga.OptionContainer(content=[home_tab])

# self.main_window.hide()
self.main_window.content = option_container
# self.main_window.show()

def main():
return TogaApp("Toga App", 'org.beeware.togaapp')

if __name__ == "__main__":
main().main_loop()
```

Here is an example of a window produced by the foregoing code:

And here is an example if you uncomment the hide and show commands:

### Steps to reproduce

1. Run the povided code on Windows and with Toga 0.5.0
2. The rendered Label and Button elements are too large.

### Expected behavior

The window produced by this code should be the same regardless of when the window is actually shown.

### Screenshots

_No response_

### Environment

- Operating System: Windows 11
- Python version: 3.12.7
- Software versions:
- Toga: 0.5.0

### Logs

```

```

### Additional context

I ran into this when reassigning the MainWindow's content. E.g., from an initial "loading" box to its final post-loading content.

Begin by running the provided reproduction on Windows with Toga 0.5.0, comparing the layout when the window is shown before assigning the OptionContainer with the hide-and-show variant. Trace the Windows widget sizing and layout lifecycle for Label and Button elements; done means both sequences produce the same-sized widgets.

Written by the indexing model from the issue text.