There should be a way to position bevy_ui children independently of parent Flex parameters

## Bevy version

main: ede5848bf

## What you did

I set the border of a node with a `PositionType::Absolute` child.

## What went wrong

The child's position changed according to the border width, while it shouldn't, as `Absolute` implies.

## Additional information

Click to see Minimal reproducible example

```rust
use bevy::prelude::*;

fn main() {
App::new()
.add_plugins(DefaultPlugins)
.add_systems(Startup, setup)
.add_systems(Update, update_border)
.run();
}
#[derive(Component)]
struct UpdateBorder;
fn update_border(
mut q: Query<&mut Style, With>,
input: Res>,
mut expanded: Local,
) {
if !input.just_pressed(KeyCode::Space) { return; }
if let Ok(mut style) = q.get_single_mut() {
let border = if *expanded { 0.0 } else { 10.0 };
style.border = UiRect::all(Val::Px(border));
*expanded = !*expanded;
}
}
fn setup(mut cmds: Commands) {
cmds.spawn(Camera2dBundle::default());
let style = TextStyle { color: Color::BLACK, ..default() };
cmds
.spawn((
NodeBundle {
border_color: Color::RED.into(),
background_color: Color::YELLOW.into(),
style: Style { width: Val::Px(300.0), height: Val::Px(50.0), ..default() },
..default()
},
UpdateBorder,
))
.with_children(|cmds| {
cmds.spawn(TextBundle {
style: Style {
position_type: PositionType::Absolute,
left: Val::Px(0.),
right: Val::Px(0.),
top: Val::Px(0.),
bottom: Val::Px(0.),
..default()
},
..TextBundle::from_section("Bad border!", style)
});
});
}
```



Reproduce the behavior using the setup and update_border entry points in the issue, toggling the border while observing the absolutely positioned child. Trace Bevy's UI Style and layout handling for PositionType::Absolute and border changes. Done means the child no longer shifts when the parent's border width changes, with the minimal example behavior verified.

Written by the indexing model from the issue text.