UI content in `gradients` and `feathers_gallery` examples doesn't fit the window

## Bevy version and features

main

## What you did

Run `gradients` or `feathers_gallery` and resize the window smaller
```
cargo r --example gradients
cargo r --example feathers_gallery --features bevy_feathers
```

## What went wrong

The UI content is cropped. I would expect the content to be scaled to fit the window:

Run `cargo r --example gradients` and `cargo r --example feathers_gallery --features bevy_feathers`, then resize the window to reproduce the cropping. Inspect the UI layout code in the two example entry points; done means the content remains visible and fits when the window is made smaller.

Written by the indexing model from the issue text.