Toggleable UI layout rounding (#16841)

# 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:

```rust
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);
        }
    }
}
```

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: UkoeHB <37489173+UkoeHB@users.noreply.github.com>

crates/bevy_ui/src/layout/mod.rs

@@ -1,7 +1,7 @@
 use crate::{
     experimental::{UiChildren, UiRootNodes},
-    BorderRadius, ComputedNode, ContentSize, DefaultUiCamera, Display, Node, Outline, OverflowAxis,
+    BorderRadius, ComputedNode, ContentSize, DefaultUiCamera, Display, LayoutConfig, Node, Outline,
-    ScrollPosition, TargetCamera, UiScale, Val,
+    OverflowAxis, ScrollPosition, TargetCamera, UiScale, Val,
 };
 use bevy_ecs::{
     change_detection::{DetectChanges, DetectChangesMut},
@@ -120,10 +120,12 @@ pub fn ui_layout_system(
         &mut ComputedNode,
         &mut Transform,
         &Node,
+        Option<&LayoutConfig>,
         Option<&BorderRadius>,
         Option<&Outline>,
         Option<&ScrollPosition>,
     )>,
+
     mut buffer_query: Query<&mut ComputedTextBlock>,
     mut font_system: ResMut<CosmicFontSystem>,
 ) {
@@ -294,6 +296,7 @@ with UI components as a child of an entity without UI components, your UI layout
                 &mut commands,
                 *root,
                 &mut ui_surface,
+                true,
                 None,
                 &mut node_transform_query,
                 &ui_children,
@@ -312,11 +315,13 @@ with UI components as a child of an entity without UI components, your UI layout
         commands: &mut Commands,
         entity: Entity,
         ui_surface: &mut UiSurface,
+        inherited_use_rounding: bool,
         root_size: Option<Vec2>,
         node_transform_query: &mut Query<(
             &mut ComputedNode,
             &mut Transform,
             &Node,
+            Option<&LayoutConfig>,
             Option<&BorderRadius>,
             Option<&Outline>,
             Option<&ScrollPosition>,
@@ -330,12 +335,17 @@ with UI components as a child of an entity without UI components, your UI layout
             mut node,
             mut transform,
             style,
+            maybe_layout_config,
             maybe_border_radius,
             maybe_outline,
             maybe_scroll_position,
         )) = node_transform_query.get_mut(entity)
         {
-            let Ok((layout, unrounded_size)) = ui_surface.get_layout(entity) else {
+            let use_rounding = maybe_layout_config
+                .map(|layout_config| layout_config.use_rounding)
+                .unwrap_or(inherited_use_rounding);
+
+            let Ok((layout, unrounded_size)) = ui_surface.get_layout(entity, use_rounding) else {
                 return;
             };
 
@@ -446,6 +456,7 @@ with UI components as a child of an entity without UI components, your UI layout
                     commands,
                     child_uinode,
                     ui_surface,
+                    use_rounding,
                     Some(viewport_size),
                     node_transform_query,
                     ui_children,
@@ -573,7 +584,7 @@ mod tests {
         let mut ui_surface = world.resource_mut::<UiSurface>();
 
         for ui_entity in [ui_root, ui_child] {
-            let layout = ui_surface.get_layout(ui_entity).unwrap().0;
+            let layout = ui_surface.get_layout(ui_entity, true).unwrap().0;
             assert_eq!(layout.size.width, WINDOW_WIDTH);
             assert_eq!(layout.size.height, WINDOW_HEIGHT);
         }
@@ -962,7 +973,7 @@ mod tests {
             let mut ui_surface = world.resource_mut::<UiSurface>();
 
             let layout = ui_surface
-                .get_layout(ui_node_entity)
+                .get_layout(ui_node_entity, true)
                 .expect("failed to get layout")
                 .0;
 
@@ -1049,7 +1060,7 @@ mod tests {
         ui_schedule.run(&mut world);
 
         let mut ui_surface = world.resource_mut::<UiSurface>();
-        let layout = ui_surface.get_layout(ui_entity).unwrap().0;
+        let layout = ui_surface.get_layout(ui_entity, true).unwrap().0;
 
         // the node should takes its size from the fixed size measure func
         assert_eq!(layout.size.width, content_size.x);
@@ -1078,7 +1089,7 @@ mod tests {
 
         // a node with a content size should have taffy context
         assert!(ui_surface.taffy.get_node_context(ui_node).is_some());
-        let layout = ui_surface.get_layout(ui_entity).unwrap().0;
+        let layout = ui_surface.get_layout(ui_entity, true).unwrap().0;
         assert_eq!(layout.size.width, content_size.x);
         assert_eq!(layout.size.height, content_size.y);
 
@@ -1091,7 +1102,7 @@ mod tests {
         assert!(ui_surface.taffy.get_node_context(ui_node).is_none());
 
         // Without a content size, the node has no width or height constraints so the length of both dimensions is 0.
-        let layout = ui_surface.get_layout(ui_entity).unwrap().0;
+        let layout = ui_surface.get_layout(ui_entity, true).unwrap().0;
         assert_eq!(layout.size.width, 0.);
         assert_eq!(layout.size.height, 0.);
     }

crates/bevy_ui/src/layout/ui_surface.rs

@@ -277,23 +277,33 @@ impl UiSurface {
     /// Does not compute the layout geometry, `compute_window_layouts` should be run before using this function.
     /// On success returns a pair consisting of the final resolved layout values after rounding
     /// and the size of the node after layout resolution but before rounding.
-    pub fn get_layout(&mut self, entity: Entity) -> Result<(taffy::Layout, Vec2), LayoutError> {
+    pub fn get_layout(
+        &mut self,
+        entity: Entity,
+        use_rounding: bool,
+    ) -> Result<(taffy::Layout, Vec2), LayoutError> {
         let Some(taffy_node) = self.entity_to_taffy.get(&entity) else {
             return Err(LayoutError::InvalidHierarchy);
         };
 
-        let layout = self
+        if use_rounding {
-            .taffy
+            self.taffy.enable_rounding();
-            .layout(*taffy_node)
+        } else {
-            .cloned()
+            self.taffy.disable_rounding();
-            .map_err(LayoutError::TaffyError)?;
+        }
+
+        let out = match self.taffy.layout(*taffy_node).cloned() {
+            Ok(layout) => {
+                self.taffy.disable_rounding();
+                let taffy_size = self.taffy.layout(*taffy_node).unwrap().size;
+                let unrounded_size = Vec2::new(taffy_size.width, taffy_size.height);
+                Ok((layout, unrounded_size))
+            }
+            Err(taffy_error) => Err(LayoutError::TaffyError(taffy_error)),
+        };
 
-        self.taffy.disable_rounding();
-        let taffy_size = self.taffy.layout(*taffy_node).unwrap().size;
-        let unrounded_size = Vec2::new(taffy_size.width, taffy_size.height);
         self.taffy.enable_rounding();
-
+        out
-        Ok((layout, unrounded_size))
     }
 }
 

crates/bevy_ui/src/ui_node.rs

@@ -2550,6 +2550,28 @@ impl Default for ShadowStyle {
     }
 }
 
+#[derive(Component, Copy, Clone, Debug, PartialEq, Reflect)]
+#[reflect(Component, Debug, PartialEq, Default)]
+#[cfg_attr(
+    feature = "serialize",
+    derive(serde::Serialize, serde::Deserialize),
+    reflect(Serialize, Deserialize)
+)]
+/// This component can be added to any UI node to modify its layout behavior.
+pub struct LayoutConfig {
+    /// If set to true the coordinates for this node and its descendents will be rounded to the nearest physical pixel.
+    /// This can help prevent visual artifacts like blurry images or semi-transparent edges that can occur with sub-pixel positioning.
+    ///
+    /// Defaults to true.
+    pub use_rounding: bool,
+}
+
+impl Default for LayoutConfig {
+    fn default() -> Self {
+        Self { use_rounding: true }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate::GridPlacement;