scroll list can't scroll in flex column with a top button

Nobody has claimed this yet.

I want to scroll the list, thats my codes

```rust
use druid::im::Vector;
use druid::widget::{Button, Flex, Label, List, Scroll};
use druid::{AppLauncher, Data, Lens, Widget, WidgetExt, WindowDesc};

pub fn run() {
let main_window = WindowDesc::new(ui_builder()).title("Clipboards");

let contents = (1..100).map(|i| format!("text 测试 {}", i)).collect();

let clipboards = Clipboards { contents };

AppLauncher::with_window(main_window)
.log_to_console()
.launch(clipboards)
.unwrap()
}

#[derive(Debug, Clone, Data, Lens)]
pub struct Clipboards {
contents: Vector,
}

fn ui_builder() -> impl Widget {
let button_text = Label::new("button").with_text_size(20.0);
let button = Button::from_label(button_text)
.on_click(|_ctx, _: &mut Clipboards, _env| println!("top click"));

let list = List::new(|| {
Label::dynamic(|content: &String, _env| content.to_string())
.with_text_size(20.0)
.on_click(|_ctx, content, _env| {
println!("click {}", content);
})
})
.with_spacing(20.0)
.center()
.expand_width()
.scroll()
.vertical()
.lens(Clipboards::contents);

/*Flex::column()
.with_child(button)
.with_default_spacer()
.with_child(list)*/
list
}
```

it can scroll

and now I want to add a button on the top

```rust
use druid::im::Vector;
use druid::widget::{Button, Flex, Label, List, Scroll};
use druid::{AppLauncher, Data, Lens, Widget, WidgetExt, WindowDesc};

pub fn run() {
let main_window = WindowDesc::new(ui_builder()).title("Clipboards");

let contents = (1..100).map(|i| format!("text 测试 {}", i)).collect();

let clipboards = Clipboards { contents };

AppLauncher::with_window(main_window)
.log_to_console()
.launch(clipboards)
.unwrap()
}

#[derive(Debug, Clone, Data, Lens)]
pub struct Clipboards {
contents: Vector,
}

fn ui_builder() -> impl Widget {
let button_text = Label::new("button").with_text_size(20.0);
let button = Button::from_label(button_text)
.on_click(|_ctx, _: &mut Clipboards, _env| println!("top click"));

let list = List::new(|| {
Label::dynamic(|content: &String, _env| content.to_string())
.with_text_size(20.0)
.on_click(|_ctx, content, _env| {
println!("click {}", content);
})
})
.with_spacing(20.0)
.center()
.expand_width()
.scroll()
.vertical()
.lens(Clipboards::contents);

Flex::column()
.with_child(button)
.with_default_spacer()
.with_child(list)
}
```

but now the list can't scroll and the scroll bar is miss

how should I do to add the button on the top and keep the list scrollable?

Start from the ui_builder function and run the two examples, comparing the standalone Scroll with the List placed in Flex::column alongside the button. Read the layout behavior of the Flex and Scroll composition shown in the issue; done means the top button remains visible while the list has a working scrollbar and can scroll through all items.

Written by the indexing model from the issue text.