Toggleable UI layout rounding

[ickshonpe]

Objective

Allow users to enable or disable layout rounding for specific UI nodes and their descendants.

Fixes #16731

Solution

New component LayoutConfig that can be added to any UiNode entity. Setting the use_rounding field of LayoutConfig determines if the Node and its descendants should be given rounded or unrounded coordinates.

Testing

Not tested this extensively but it seems to work and it's not very complicated.
This really basic test app returns fractional coords:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, report)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn(Camera2d);
    commands.spawn((
        Node {
            left: Val::Px(0.1),
            width: Val::Px(100.1),
            height: Val::Px(100.1),
            ..Default::default()
        },
        LayoutConfig { use_rounding: false },
    ));
}

fn report(node: Query<(Ref<ComputedNode>, &GlobalTransform)>) {
    for (c, g) in node.iter() {
        if c.is_changed() {
            println!("{:#?}", c);
            println!("position = {:?}", g.to_scale_rotation_translation().2);
        }
    }
}

[alice-i-cecile]

@MatrixDev can I get your review here please?

[MatrixDev]

@MatrixDev can I get your review here please?

@alice-i-cecile, sure, I'll try to do it later today.

[alice-i-cecile]

Sad to add a branch here, but the use case in the linked issue is very reasonable.

[MatrixDev]

@alice-i-cecile, I can confirm that it becomes significantly smoother with this change and no longer pixel-aligned.

With use_rounding: false:

With use_rounding: true:

There is still one problem with bars but I don't know if it is related or not. All three bars (red, blue and yellow) are inside of the column with zero row_gap/colum_gap and ideally those should always stick to each other but sometimes there are gaps shown between them (in both with or without rounding):

In any case, from my point of view, original issue is resolved now.

[JeanMertz]

Any reason to add this as a field to the new LayoutConfig struct, as opposed to a newtype struct (or enum) specifically for this purpose? Do we expect more layout options to be configurable soon?

[ickshonpe]

Sad to add a branch here, but the use case in the linked issue is very reasonable.

Yeah taffy's rounding api is really unpleasant with how this setting is used both to enable or disable rounding during updates, and then afterwards to access the unrounded values. I'll make a taffy PR as well with some alternative then we'd be able to clean this up too later.

[ickshonpe]

Any reason to add this as a field to the new LayoutConfig struct, as opposed to a newtype struct (or enum) specifically for this purpose? Do we expect more layout options to be configurable soon?

I'm not really sure. I'm not against using an enum or an UnroundedLayout marker component or something instead. I guess I think if we did have any other similar niche configuration options it'd be preferable to consolidate them together in a single struct like this for the sake of discoverability. If there were other settings we'd have to add an inherit from parent option to the use_rounding setting.