[Merged by Bors] - change the default width and height of Size to Val::Auto

[ickshonpe]

Objective

In CSS Flexbox width and height are auto by default, whereas in Bevy their default is Size::Undefined.
This means that, unlike in CSS, if you elide a height or width value for a node it will be given zero length (unless it has an explicitly sized child node). This has misled users into falsely assuming that they have to explicitly set a value for both height and width all the time.

relevant issue: #7120

Solution

Change the Size width and height default values to Val::Auto

Changelog

• Changed the Size width and height default values to Val::Auto

Migration Guide

The default values for Size width and height have been changed from Val::Undefined to Val::Auto.
It's unlikely to cause any issues with existing code.

[nicoburns]

This change seems unproblematic to me. However, I believe it will have no effect as IIRC Taffy versions that have both Undefined and Auto treat them both as Auto anyway.

[ickshonpe]

Undefined and Auto definitely aren't the same.

Ultra minimal example, red window:

use bevy::prelude::*;
fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(|mut commands: Commands| {
            commands.spawn(Camera2dBundle::default());
            commands.spawn(NodeBundle {
                style: Style {
                    size: Size::new(Val::Percent(100.), Val::Auto),
                    ..Default::default()
                },
                background_color: BackgroundColor(Color::RED),
                ..Default::default()
            });
        })
        .run();
}

Grey window:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(|mut commands: Commands| {
            commands.spawn(Camera2dBundle::default());
            commands.spawn(NodeBundle {
                style: Style {
                    size: Size::new(Val::Percent(100.), Val::Undefined),
                    ..Default::default()
                },
                background_color: BackgroundColor(Color::RED),
                ..Default::default()
            });
        })
        .run();

The Taffy 2.x compute source is full of predicates like child_style.cross_size(constants.dir) == Dimension::Auto etc.

[alice-i-cecile]

bors r+

[bors]

Build failed (retrying...):

• build-android

[bors]

Pull request successfully merged into main.

Build succeeded:

• build-and-install-on-iOS
• build-android
• build (macos-latest)
• build (ubuntu-latest)
• build-wasm
• build (windows-latest)
• build-without-default-features (bevy)
• build-without-default-features (bevy_ecs)
• build-without-default-features (bevy_reflect)
• check-compiles
• check-doc
• check-missing-examples-in-docs
• ci
• markdownlint
• msrv
• run-examples
• run-examples-on-wasm
• run-examples-on-windows-dx12