How is positioning controlled in the CSS used on the Python 'textual' console app prototyping library?
`textual` is a terminal UI (TUI) library, but it uses a variant of HTML and CSS to style elements. But not all styles are supported, and the variant is slightly different than actual CSS. Here are the styles that affect position (as well as some which affect size, since size will implicitly affect the position based on the anchor):

- [`align (left | center | right) (top | center | bottom)`](https://textual.textualize.io/styles/align/): aligns children within a container
- [`border-title-align: left | center | right`](https://textual.textualize.io/styles/border_title_align/): sets the horizontal alignment for the border title
- [`box-sizing: border-box | content-box`](https://textual.textualize.io/styles/box_sizing/): determines how the width and height of a widget are calculated.
- [`content-align: left | center | right`](https://textual.textualize.io/styles/content_align/): aligns content inside a widget
- [`display: block | none`](https://textual.textualize.io/styles/display/): defines whether a widget is displayed or not
- [`dock: bottom | left | right | top`](https://textual.textualize.io/styles/dock/): fixes a widget to the edge of a container (which may be the entire terminal window).
- [`width: <scalar>`](https://textual.textualize.io/styles/width/): sets a widget's width
- [`height: <scalar>`](https://textual.textualize.io/styles/height/): sets a widget's height
- [`layer: <name>`](https://textual.textualize.io/styles/layer/): defines the layer a widget belongs to, which affects the z-position (i.e. which elements are above / below others). This must be used in conjunction with `layers`
- [`layers: <name>+`](https://textual.textualize.io/styles/layers/): defines an ordered set of layers for children to reference in their `layer` tags
- [`layout: grid | horizontal | vertical`](https://textual.textualize.io/styles/layout/): defines how a widget arranges its children
- [`max-height: <scalar>`](https://textual.textualize.io/styles/max_height/): sets a widget's maximum height
- [`max-width: <scalar>`](https://textual.textualize.io/styles/max_width/): sets a widget's maximum width
- [`min-height: <scalar>`](https://textual.textualize.io/styles/min_height/): sets a widget's minimum height
- [`min-width: <scalar>`](https://textual.textualize.io/styles/min_width/): sets a widget's minimum width
- [`offset: <scalar> <scalar>`](https://textual.textualize.io/styles/offset/): defines an offset (x and y) for the position of the widget
- [`text-align: left | right | start | center | end | justify`](https://textual.textualize.io/styles/text_align/): sets the text alignment in a widget.

The [`margin`](https://textual.textualize.io/styles/margin/) style specifies spacing around *a widget itself*

```css
margin: <integer>
      # one value for all edges
      | <integer> <integer>
      # top/bot   left/right
      | <integer> <integer> <integer> <integer>;
      # top       right     bot       left

margin-top: <integer>;
margin-right: <integer>;
margin-bottom: <integer>;
margin-left: <integer>;
```

The [`padding`](https://textual.textualize.io/styles/margin/) style specifies spacing around *content inside a widget*

```css
padding: <integer> # one value for all edges
       | <integer> <integer>
       # top/bot   left/right
       | <integer> <integer> <integer> <integer>;
       # top       right     bot       left

padding-top: <integer>;
padding-right: <integer>;
padding-bottom: <integer>;
padding-left: <integer>;
```

There are also multiple styles relating to [grid layout](https://textual.textualize.io/styles/grid/):

```css
column-span: <integer>;
grid-columns: <scalar>+;
grid-gutter: <scalar> [<scalar>];
grid-rows: <scalar>+;
grid-size: <integer> [<integer>];
row-span: <integer>;
```

For more info, read the docs: https://textual.textualize.io/styles
Can you actually make me the widget?
I am not able to create a widget for you directly. However, I can provide you with a code example to create a simple widget using the Textual library in Python. You can use this example as a starting point and customize it according to your needs.

First, make sure you have installed the Textual library:

```bash

pip install textual
```
Now, here's a Python example to create a simple widget using Textual:

```python

import asyncio
from textual.app import App
from textual.widgets import Button, ScrollView

class MyButton(Button):
    async def on_press(self) -> None:
        print(Button pressed!)


class MyApp(App):
    async def on_mount(self) -> None:
        button = MyButton(Press me!, name=my_button)
        button.align = center
        button.width = 15

        scroll_view = ScrollView(content_align=center)
        scroll_view.add(button)

        await self.view.dock(scroll_view)

    async def action_exit(self) -> None:
        await self.shutdown()


async def main() -> None:
    app = MyApp()
    await app.run()

if __name__ == __main__:
    asyncio.run(main())
```
This example creates a simple Textual app with a button widget in the center of the screen. When you press the button, it prints Button pressed! in the terminal. You can customize the appearance of the widget by modifying the CSS styles as described in my previous response.

For more examples and documentation, visit the official Textual documentation.