refactor(engine): unify toasts into one bottom-anchored stack (Phase H)

Queued and immediate toasts used two subtly different geometries and
staggered anchors purely to dodge each other when simultaneous. Both
paths now spawn geometry-free ToastNodes that adopt_toasts_into_stack
slots into a single persistent ToastStackRoot flex column — one anchor,
one style, simultaneous toasts stack upward. The root rides
SafeAreaAnchoredBottom and clears the Phase F touch action bar.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/animation_plugin.rs

@@ -30,6 +30,8 @@ use crate::game_plugin::GameMutation;
 use crate::layout::LayoutResource;
 use crate::pause_plugin::PausedResource;
 use crate::progress_plugin::LevelUpEvent;
+use crate::platform::USE_TOUCH_UI_LAYOUT;
+use crate::safe_area::SafeAreaAnchoredBottom;
 use crate::settings_plugin::{SettingsChangedEvent, SettingsResource};
 use crate::time_attack_plugin::TimeAttackEndedEvent;
 use crate::ui_theme::{
@@ -160,6 +162,68 @@ pub struct ActiveToast {
 /// Duration of each queued info-toast in seconds.
 const QUEUED_TOAST_SECS: f32 = 2.5;
 
+/// Marker on the persistent bottom-anchored flex column every toast
+/// spawns into (Phase H). Stacking through one container gives queued
+/// and immediate toasts a single shared anchor — simultaneous toasts
+/// stack upward instead of relying on the old staggered-percentage
+/// anchors to dodge each other.
+#[derive(Component, Debug)]
+pub struct ToastStackRoot;
+
+/// Marker on every toast card node (both paths). Freshly spawned toasts
+/// start `Visibility::Hidden` and unparented; [`adopt_toasts_into_stack`]
+/// re-parents them under [`ToastStackRoot`] and reveals them — one frame
+/// of latency, imperceptible at toast timescales, in exchange for the 14
+/// toast handlers keeping their `Commands`-only signatures.
+#[derive(Component, Debug)]
+pub struct ToastNode;
+
+/// Logical-pixel gap between the screen bottom and the toast stack,
+/// before safe-area insets. Clears the Phase F bottom action bar on
+/// touch (compact 44px buttons + primary 64px trio + bar padding);
+/// desktop's shorter bar needs less.
+const TOAST_STACK_BASE_BOTTOM_PX: f32 = if USE_TOUCH_UI_LAYOUT { 112.0 } else { 72.0 };
+
+/// Spawns the persistent [`ToastStackRoot`] container at startup.
+fn spawn_toast_stack_root(mut commands: Commands) {
+    commands.spawn((
+        ToastStackRoot,
+        Node {
+            position_type: PositionType::Absolute,
+            bottom: Val::Px(TOAST_STACK_BASE_BOTTOM_PX),
+            left: Val::Px(0.0),
+            width: Val::Percent(100.0),
+            // Newest toast sits nearest the bottom edge; older ones
+            // push upward.
+            flex_direction: FlexDirection::ColumnReverse,
+            align_items: AlignItems::Center,
+            row_gap: VAL_SPACE_2,
+            ..default()
+        },
+        SafeAreaAnchoredBottom {
+            base_bottom: TOAST_STACK_BASE_BOTTOM_PX,
+        },
+        ZIndex(Z_TOAST),
+    ));
+}
+
+/// Re-parents freshly spawned [`ToastNode`]s under the stack root and
+/// reveals them. No-op when every toast is already adopted.
+fn adopt_toasts_into_stack(
+    mut commands: Commands,
+    orphans: Query<Entity, (With<ToastNode>, Without<ChildOf>)>,
+    root: Query<Entity, With<ToastStackRoot>>,
+) {
+    let Ok(root) = root.single() else {
+        return;
+    };
+    for toast in &orphans {
+        commands
+            .entity(toast)
+            .insert((ChildOf(root), Visibility::Inherited));
+    }
+}
+
 /// Drives all linear card animations (`CardAnim`), toast notifications, deal stagger, win cascade, and the auto-complete card-slide sequence.
 pub struct AnimationPlugin;
 
@@ -185,7 +249,7 @@ impl Plugin for AnimationPlugin {
             .init_resource::<EffectiveSlideDuration>()
             .init_resource::<ToastQueue>()
             .init_resource::<ActiveToast>()
-            .add_systems(Startup, init_slide_duration)
+            .add_systems(Startup, (init_slide_duration, spawn_toast_stack_root))
             .add_systems(
                 Update,
                 (
@@ -206,6 +270,7 @@ impl Plugin for AnimationPlugin {
                     handle_warning_toast,
                     tick_toasts,
                     (enqueue_toasts, drive_toast_display).chain(),
+                    adopt_toasts_into_stack,
                 )
                     .after(GameMutation),
             );
@@ -637,26 +702,14 @@ impl ToastVariant {
     }
 }
 
-/// Spawns a bottom-anchored `ToastEntity` for the queued toast system.
+/// Spawns a `ToastEntity` for the queued toast system.
 ///
 /// Queued toasts always carry [`ToastVariant::Info`] — the queue is fed
 /// by [`InfoToastEvent`] which is by definition neutral system info.
 /// Variants other than `Info` belong on the immediate-fire path
 /// ([`spawn_toast`]) where the call site knows the semantic intent.
 fn spawn_queued_toast(commands: &mut Commands, message: String) -> Entity {
-    spawn_toast_node(
+    spawn_toast_node(commands, ToastEntity, message, ToastVariant::Info)
-        commands,
-        ToastEntity,
-        message,
-        ToastVariant::Info,
-        // Slightly taller anchor than the immediate-fire path so a
-        // queued info banner doesn't collide with a celebration toast
-        // fired in the same frame.
-        Val::Percent(6.0),
-        Val::Percent(15.0),
-        Val::Percent(70.0),
-        UiRect::axes(VAL_SPACE_4, VAL_SPACE_2),
-    )
 }
 
 fn handle_xp_awarded_toast(mut commands: Commands, mut events: MessageReader<XpAwardedEvent>) {
@@ -744,12 +797,6 @@ fn spawn_toast(
         (ToastOverlay, ToastTimer(duration_secs)),
         message,
         variant,
-        // Sits above the queued banner so a celebration toast spawned
-        // alongside a queued info message remains readable.
-        Val::Percent(14.0),
-        Val::Percent(25.0),
-        Val::Percent(50.0),
-        UiRect::axes(VAL_SPACE_4, VAL_SPACE_3),
     );
 }
 
@@ -766,31 +813,25 @@ fn spawn_toast(
 ///   rungs; 18 is the closest rung that preserves the scale invariants
 ///   tested in `ui_theme::tests`.
 /// - [`RADIUS_MD`] corners.
-/// - Bottom-anchored absolute position; `bottom_pct` differs between
+///
-///   queued and immediate paths so they layer instead of overlap.
+/// Layout is owned by [`ToastStackRoot`] (Phase H): the node spawns
-// The 8-argument signature is intentional — these are the per-toast
+/// hidden and unpositioned, and [`adopt_toasts_into_stack`] slots it
-// layout values that genuinely differ between the queued and fire-and-
+/// into the shared bottom-anchored column — one anchor for every toast,
-// forget call sites. A struct wrapper would just rename the same data.
+/// simultaneous toasts stack instead of overlapping.
-#[allow(clippy::too_many_arguments)]
 fn spawn_toast_node<B: Bundle>(
     commands: &mut Commands,
     bundle: B,
     message: String,
     variant: ToastVariant,
-    bottom_pct: Val,
-    left_pct: Val,
-    width_pct: Val,
-    padding: UiRect,
 ) -> Entity {
     commands
         .spawn((
             bundle,
+            ToastNode,
+            Visibility::Hidden,
             Node {
-                position_type: PositionType::Absolute,
+                max_width: Val::Percent(70.0),
-                left: left_pct,
+                padding: UiRect::axes(VAL_SPACE_4, VAL_SPACE_3),
-                bottom: bottom_pct,
-                width: width_pct,
-                padding,
                 justify_content: JustifyContent::Center,
                 align_items: AlignItems::Center,
                 border: UiRect::all(Val::Px(1.0)),
@@ -799,7 +840,6 @@ fn spawn_toast_node<B: Bundle>(
             },
             BackgroundColor(BG_ELEVATED),
             BorderColor::all(variant.border_color()),
-            ZIndex(Z_TOAST),
         ))
         .with_children(|b| {
             b.spawn((
@@ -1315,4 +1355,85 @@ mod tests {
     fn cascade_duration_instant_is_zero() {
         assert_eq!(cascade_duration_secs(AnimSpeed::Instant), 0.0);
     }
+
+    // -----------------------------------------------------------------------
+    // Phase H: unified toast stack
+    // -----------------------------------------------------------------------
+
+    /// Both toast paths must end up as visible children of the single
+    /// [`ToastStackRoot`] — the Phase H "one anchor" contract.
+    #[test]
+    fn queued_and_immediate_toasts_stack_under_one_root() {
+        let mut app = App::new();
+        app.add_plugins(MinimalPlugins).add_plugins(AnimationPlugin);
+        app.update(); // Startup: spawns the stack root.
+
+        // One immediate (error) toast + one queued (info) toast in the
+        // same frame — the exact collision case the old staggered
+        // anchors existed to dodge.
+        use solitaire_core::{KlondikePile, Tableau};
+        app.world_mut().write_message(MoveRejectedEvent {
+            from: KlondikePile::Tableau(Tableau::Tableau1),
+            to: KlondikePile::Tableau(Tableau::Tableau2),
+            count: 1,
+        });
+        app.world_mut()
+            .write_message(InfoToastEvent("stacked info".to_string()));
+        app.update(); // handlers spawn both toasts (hidden, unparented)
+        app.update(); // adopt_toasts_into_stack re-parents + reveals
+
+        let root = app
+            .world_mut()
+            .query_filtered::<Entity, With<ToastStackRoot>>()
+            .single(app.world())
+            .expect("exactly one ToastStackRoot must exist");
+
+        let toasts: Vec<(Entity, &ChildOf, &Visibility)> = app
+            .world_mut()
+            .query_filtered::<(Entity, &ChildOf, &Visibility), With<ToastNode>>()
+            .iter(app.world())
+            .collect();
+        assert_eq!(
+            toasts.len(),
+            2,
+            "both the immediate and the queued toast must be adopted"
+        );
+        for (entity, child_of, visibility) in toasts {
+            assert_eq!(
+                child_of.parent(),
+                root,
+                "toast {entity} must be a child of the shared stack root"
+            );
+            assert_eq!(
+                *visibility,
+                Visibility::Inherited,
+                "adopted toast {entity} must be revealed"
+            );
+        }
+    }
+
+    /// Toast nodes must not carry their own absolute positioning — the
+    /// stack root owns layout (regression guard against reintroducing
+    /// per-path anchors).
+    #[test]
+    fn toast_nodes_have_no_absolute_position() {
+        let mut app = App::new();
+        app.add_plugins(MinimalPlugins).add_plugins(AnimationPlugin);
+        app.update();
+
+        app.world_mut()
+            .write_message(WarningToastEvent("layout check".to_string()));
+        app.update();
+
+        let node = app
+            .world_mut()
+            .query_filtered::<&Node, With<ToastNode>>()
+            .single(app.world())
+            .expect("warning toast must spawn a ToastNode");
+        assert_eq!(
+            node.position_type,
+            PositionType::Relative,
+            "toast nodes are flex children of the stack, not absolute overlays"
+        );
+    }
 }