feat(engine): replace docked touch bar with floating glass tab bar

Touch layout only (desktop bar unchanged): a floating glass pill
(Undo / Draw / Hint / Pause) plus a detached circular Menu button,
margin above the bottom edge on top of the safe-area inset. Draw is
the persistently expanded accent pill (icon + label); other buttons
are icon-only and slide their label out while pressed, snapping under
reduce-motion. Buttons keep the existing marker components so click
handlers, tooltips, focus ring, and chrome toggle work unchanged.
Icons: None degrades to text-fallback labels (MinimalPlugins tests /
SVG regression), keeping the bar fully usable.

Toast stack clearance now derives from TAB_BAR_CLEARANCE_PX; the
legacy touch bar spawn path and primary-button metrics are removed.

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

solitaire_engine/src/hud_plugin/tab_bar.rs

@@ -0,0 +1,470 @@
+//! Floating "liquid glass" touch tab bar — layout and animation.
+//!
+//! Replaces the docked full-width touch action bar (Phase F) with an
+//! iOS-26-style floating arrangement: a pill of icon buttons hovering
+//! above the bottom edge, plus a detached circular Menu button to its
+//! right. The *visual treatment* (translucent sheen, specular rim, drop
+//! shadow) lives in [`crate::ui_glass`]; this module owns only geometry,
+//! button wiring, and the expand/collapse label animation, so the glass
+//! material can be swapped without touching layout.
+//!
+//! Interaction compatibility: every button keeps the same marker
+//! components as the old bar (`UndoButton`, `DrawButton`, …) plus
+//! `ActionButton`, `Button`, `Tooltip`, and `Focusable`, so the click
+//! handlers, tooltips, focus ring, and the tap-to-toggle chrome guard in
+//! `interaction.rs` all work unchanged.
+//!
+//! Expansion model (user decision, 2026-07-15): **Draw** — the most
+//! frequent action — is the persistently-expanded "active" pill showing
+//! icon + label; the other buttons are icon-only and reveal their label
+//! with a width-animated slide while pressed.
+
+use super::*;
+use crate::assets::hud_icon_svg::HudIcon;
+use crate::ui_glass::{GLASS_BORDER_PX, glass_surface};
+use crate::ui_theme::{ACCENT_PRIMARY_HOVER, SPACE_3};
+use bevy::text::{LineBreak, TextLayout};
+use bevy::ui::Overflow;
+
+/// Height of both the pill and the circular Menu button, in logical px.
+/// 64 keeps the inner 48-px buttons comfortably past the 44-px touch
+/// floor with 8 px of glass around them.
+pub(crate) const TAB_BAR_HEIGHT: f32 = 64.0;
+
+/// Gap between the bar and the bottom screen edge (the "floating" margin;
+/// the Android gesture-bar inset is added on top by `SafeAreaAnchoredBottom`).
+const TAB_BAR_MARGIN: f32 = SPACE_3;
+
+/// Vertical space UI stacked above the bar (the toast stack) must clear:
+/// floating margin + bar height + one spacing step of breathing room.
+pub(crate) const TAB_BAR_CLEARANCE_PX: f32 = TAB_BAR_MARGIN + TAB_BAR_HEIGHT + SPACE_2;
+
+/// Diameter of a button inside the pill (and its fully-round radius ×2).
+const TAB_BUTTON_SIZE: f32 = 48.0;
+
+/// Displayed icon size in logical px (texture is rasterised larger; see
+/// `assets::hud_icon_svg::HUD_ICON_TEXTURE_PX`).
+const TAB_ICON_PX: f32 = 24.0;
+
+/// FiraMono glyph advance as a fraction of font size. The bar is
+/// monospace-only, so `chars × advance × font_size` is the exact label
+/// width — the same ratio `action_bar_font_size` documents.
+const FIRA_ADVANCE: f32 = 0.62;
+
+/// Gap between an icon and its revealed label, folded into the animated
+/// wrapper width so it collapses together with the text.
+const LABEL_LEAD_PX: f32 = SPACE_2;
+
+/// Idle fill of an inactive icon button — fully transparent so the pill's
+/// glass shows through untouched.
+const GLASS_BTN_IDLE: Color = Color::srgba(1.0, 1.0, 1.0, 0.0);
+/// Hover fill of an inactive icon button (desktop pointer / DPAD focus).
+const GLASS_BTN_HOVER: Color = Color::srgba(1.0, 1.0, 1.0, 0.08);
+/// Pressed fill of an inactive icon button.
+const GLASS_BTN_PRESSED: Color = Color::srgba(1.0, 1.0, 1.0, 0.14);
+/// Pressed state of the active (accent-filled) pill — darker than idle so
+/// the press reads as the surface receding, mirroring `BG_ELEVATED_PRESSED`.
+const GLASS_ACTIVE_PRESSED: Color = Color::srgb(0.55, 0.22, 0.22);
+
+/// Idle / hovered / pressed fills for one tab-bar button. Presence of this
+/// component also routes the button to `paint_glass_tab_buttons` instead of
+/// the legacy `paint_action_buttons` (which excludes it).
+#[derive(Component, Debug)]
+pub struct GlassTabButton {
+    idle: Color,
+    hovered: Color,
+    pressed: Color,
+}
+
+/// Animated label-reveal state, on the clipping wrapper node between the
+/// button and its `Text` label. `progress` ∈ [0, 1] maps to wrapper width
+/// via [`expansion_width`]; `target` is where it is heading.
+#[derive(Component, Debug)]
+pub struct TabLabelWrap {
+    /// Current reveal amount, 0 = collapsed, 1 = fully expanded.
+    pub progress: f32,
+    /// Reveal amount the animation is moving towards.
+    pub target: f32,
+    /// `true` keeps the label expanded regardless of interaction —
+    /// the persistent Draw pill, and every button in text-fallback mode.
+    persistent: bool,
+    /// Label length; width is recomputed from the live font size each
+    /// frame so viewport-driven font rescales stay correct.
+    label_chars: usize,
+    /// Last width written to the node, to skip no-op layout writes.
+    last_width: f32,
+}
+
+/// Marker on the label `Text` inside a [`TabLabelWrap`].
+#[derive(Component, Debug)]
+pub struct TabLabel;
+
+/// Rasterised icon textures for the five tab-bar actions.
+pub(super) struct TabIcons {
+    undo: Handle<Image>,
+    draw: Handle<Image>,
+    hint: Handle<Image>,
+    pause: Handle<Image>,
+    menu: Handle<Image>,
+}
+
+impl TabIcons {
+    /// Rasterise all five icons into `images`. Returns `None` (and warns)
+    /// if any icon fails — the caller then falls back to text-only labels,
+    /// so a bad SVG degrades the look, never the playability.
+    pub(super) fn rasterize_into(images: &mut Assets<Image>) -> Option<Self> {
+        let mut add = |icon: HudIcon| match icon.rasterize() {
+            Ok(img) => Some(images.add(img)),
+            Err(e) => {
+                warn!("tab_bar: failed to rasterise {icon:?} icon: {e}");
+                None
+            }
+        };
+        Some(Self {
+            undo: add(HudIcon::Undo)?,
+            draw: add(HudIcon::Draw)?,
+            hint: add(HudIcon::Hint)?,
+            pause: add(HudIcon::Pause)?,
+            menu: add(HudIcon::Menu)?,
+        })
+    }
+}
+
+/// Wrapper width in logical px for a label of `chars` monospace glyphs at
+/// `font_size`, revealed to `progress`. Pure so the animation curve is
+/// unit-testable without an app.
+pub(super) fn expansion_width(chars: usize, font_size: f32, progress: f32) -> f32 {
+    progress.clamp(0.0, 1.0) * (LABEL_LEAD_PX + chars as f32 * font_size * FIRA_ADVANCE)
+}
+
+/// Advance `progress` towards `target` by one frame of `dt` seconds.
+/// Linear tween over `MOTION_SLIDE_SECS`, snapping when reduce-motion is
+/// active. Pure for the same reason as [`expansion_width`].
+pub(super) fn step_expansion(progress: f32, target: f32, dt: f32, reduce_motion: bool) -> f32 {
+    if reduce_motion {
+        return target;
+    }
+    let max_step = dt / crate::ui_theme::MOTION_SLIDE_SECS;
+    progress + (target - progress).clamp(-max_step, max_step)
+}
+
+/// Spawns the floating glass tab bar: one full-width invisible flex row
+/// (carrying the `HudActionBar` marker so chrome show/hide keeps working)
+/// centring the glass pill and the detached circular Menu button.
+///
+/// `icons: None` (rasteriser unavailable, e.g. `MinimalPlugins` tests, or
+/// an SVG regression) switches every button to text-fallback: labels stay
+/// expanded so the bar remains fully usable without textures.
+pub(super) fn spawn_glass_tab_bar(
+    commands: &mut Commands,
+    font: &TextFont,
+    icons: Option<&TabIcons>,
+) {
+    let text_fallback = icons.is_none();
+    commands
+        .spawn((
+            Node {
+                position_type: PositionType::Absolute,
+                bottom: Val::Px(TAB_BAR_MARGIN),
+                left: Val::Px(0.0),
+                width: Val::Percent(100.0),
+                flex_direction: FlexDirection::Row,
+                justify_content: JustifyContent::Center,
+                align_items: AlignItems::Center,
+                column_gap: VAL_SPACE_3,
+                ..default()
+            },
+            ZIndex(Z_HUD),
+            SafeAreaAnchoredBottom {
+                base_bottom: TAB_BAR_MARGIN,
+            },
+            HudActionBar,
+        ))
+        .with_children(|bar| {
+            // The main pill: Undo · [Draw] · Hint · Pause.
+            bar.spawn((
+                Node {
+                    height: Val::Px(TAB_BAR_HEIGHT),
+                    flex_direction: FlexDirection::Row,
+                    align_items: AlignItems::Center,
+                    padding: UiRect::axes(Val::Px(SPACE_2), Val::Px(0.0)),
+                    column_gap: Val::Px(4.0),
+                    border: UiRect::all(Val::Px(GLASS_BORDER_PX)),
+                    border_radius: BorderRadius::all(Val::Px(TAB_BAR_HEIGHT / 2.0)),
+                    ..default()
+                },
+                glass_surface(),
+            ))
+            .with_children(|pill| {
+                spawn_tab_button(
+                    pill,
+                    UndoButton,
+                    "Undo",
+                    "Take back your last move. Hold to keep undoing. Costs points and blocks No Undo.",
+                    font,
+                    0,
+                    icons.map(|i| i.undo.clone()),
+                    false,
+                    text_fallback,
+                );
+                spawn_tab_button(
+                    pill,
+                    DrawButton,
+                    "Draw",
+                    "Draw from the stock — same as tapping the deck.",
+                    font,
+                    1,
+                    icons.map(|i| i.draw.clone()),
+                    true,
+                    text_fallback,
+                );
+                spawn_tab_button(
+                    pill,
+                    HintButton,
+                    TOUCH_HINT_LABEL,
+                    "Highlight a suggested move. Cycles through alternatives on repeat taps.",
+                    font,
+                    2,
+                    icons.map(|i| i.hint.clone()),
+                    false,
+                    text_fallback,
+                );
+                spawn_tab_button(
+                    pill,
+                    PauseButton,
+                    "Pause",
+                    "Pause the game and freeze the timer.",
+                    font,
+                    3,
+                    icons.map(|i| i.pause.clone()),
+                    false,
+                    text_fallback,
+                );
+            });
+
+            // The detached circular Menu button, same glass material.
+            bar.spawn((
+                MenuButton,
+                ActionButton,
+                Button,
+                GlassTabButton {
+                    idle: GLASS_BTN_IDLE,
+                    hovered: GLASS_BTN_HOVER,
+                    pressed: GLASS_BTN_PRESSED,
+                },
+                Tooltip::new(
+                    "Open Home, Stats, Achievements, Profile, Settings, or Leaderboard.",
+                ),
+                Focusable {
+                    group: FocusGroup::Hud,
+                    order: 4,
+                },
+                Node {
+                    width: Val::Px(TAB_BAR_HEIGHT),
+                    height: Val::Px(TAB_BAR_HEIGHT),
+                    justify_content: JustifyContent::Center,
+                    align_items: AlignItems::Center,
+                    border: UiRect::all(Val::Px(GLASS_BORDER_PX)),
+                    border_radius: BorderRadius::all(Val::Px(TAB_BAR_HEIGHT / 2.0)),
+                    ..default()
+                },
+                BackgroundColor(GLASS_BTN_IDLE),
+                glass_surface(),
+            ))
+            .with_children(|circle| {
+                if let Some(icons) = icons {
+                    spawn_tab_icon(circle, icons.menu.clone(), TEXT_PRIMARY);
+                } else {
+                    // Text fallback: a menu glyph-free label keeps the
+                    // button discoverable without any texture.
+                    circle.spawn((
+                        Text::new("Menu"),
+                        TextLayout::new_with_linebreak(LineBreak::NoWrap),
+                        font.clone(),
+                        TextColor(TEXT_PRIMARY),
+                    ));
+                }
+            });
+        });
+}
+
+/// Spawns one button inside the pill: icon, then the width-animated label
+/// wrapper. `active` gives the accent-filled persistent pill (Draw);
+/// `text_fallback` pins every label open when icons are unavailable.
+#[allow(clippy::too_many_arguments)]
+fn spawn_tab_button<M: Component>(
+    pill: &mut ChildSpawnerCommands,
+    marker: M,
+    label: &str,
+    tooltip: &'static str,
+    font: &TextFont,
+    order: i32,
+    icon: Option<Handle<Image>>,
+    active: bool,
+    text_fallback: bool,
+) {
+    let (idle, hovered, pressed) = if active {
+        (ACCENT_PRIMARY, ACCENT_PRIMARY_HOVER, GLASS_ACTIVE_PRESSED)
+    } else {
+        (GLASS_BTN_IDLE, GLASS_BTN_HOVER, GLASS_BTN_PRESSED)
+    };
+    let icon_tint = if active { TEXT_PRIMARY } else { TEXT_SECONDARY };
+    let persistent = active || text_fallback;
+    let start = if persistent { 1.0 } else { 0.0 };
+    let chars = label.chars().count();
+
+    pill.spawn((
+        marker,
+        ActionButton,
+        Button,
+        GlassTabButton {
+            idle,
+            hovered,
+            pressed,
+        },
+        Tooltip::new(tooltip),
+        Focusable {
+            group: FocusGroup::Hud,
+            order,
+        },
+        Node {
+            height: Val::Px(TAB_BUTTON_SIZE),
+            min_width: Val::Px(TAB_BUTTON_SIZE),
+            flex_direction: FlexDirection::Row,
+            justify_content: JustifyContent::Center,
+            align_items: AlignItems::Center,
+            padding: UiRect::axes(Val::Px(SPACE_3), Val::Px(0.0)),
+            border_radius: BorderRadius::all(Val::Px(TAB_BUTTON_SIZE / 2.0)),
+            ..default()
+        },
+        BackgroundColor(idle),
+    ))
+    .with_children(|b| {
+        if let Some(handle) = icon {
+            spawn_tab_icon(b, handle, icon_tint);
+        }
+        // The clipping wrapper the animation drives. Label text sits
+        // left-anchored inside, so growing the width reveals it
+        // left-to-right and shrinking clips it from the right.
+        b.spawn((
+            TabLabelWrap {
+                progress: start,
+                target: start,
+                persistent,
+                label_chars: chars,
+                last_width: f32::NAN,
+            },
+            Node {
+                overflow: Overflow::clip(),
+                width: Val::Px(expansion_width(chars, font.font_size, start)),
+                height: Val::Percent(100.0),
+                align_items: AlignItems::Center,
+                justify_content: JustifyContent::FlexEnd,
+                ..default()
+            },
+        ))
+        .with_children(|wrap| {
+            let mut color = TEXT_PRIMARY;
+            color.set_alpha(start);
+            wrap.spawn((
+                TabLabel,
+                ActionButtonLabel,
+                Text::new(label),
+                TextLayout::new_with_linebreak(LineBreak::NoWrap),
+                font.clone(),
+                TextColor(color),
+            ));
+        });
+    });
+}
+
+/// One fixed-size tinted icon node.
+fn spawn_tab_icon(parent: &mut ChildSpawnerCommands, handle: Handle<Image>, tint: Color) {
+    parent.spawn((
+        ImageNode {
+            color: tint,
+            ..ImageNode::new(handle)
+        },
+        Node {
+            width: Val::Px(TAB_ICON_PX),
+            height: Val::Px(TAB_ICON_PX),
+            flex_shrink: 0.0,
+            ..default()
+        },
+    ));
+}
+
+/// Sets each label wrapper's animation target from its button's
+/// interaction state: persistent wrappers pin to 1, the rest expand while
+/// pressed or hovered and collapse on release.
+pub(super) fn drive_tab_expansion(
+    buttons: Query<(&Interaction, &Children), With<GlassTabButton>>,
+    mut wraps: Query<&mut TabLabelWrap>,
+) {
+    for (interaction, children) in &buttons {
+        for child in children.iter() {
+            let Ok(mut wrap) = wraps.get_mut(child) else {
+                continue;
+            };
+            let target = if wrap.persistent || !matches!(interaction, Interaction::None) {
+                1.0
+            } else {
+                0.0
+            };
+            // Bypass change detection on no-ops so `animate_tab_expansion`
+            // and layout stay idle while nothing moves.
+            if wrap.target != target {
+                wrap.target = target;
+            }
+        }
+    }
+}
+
+/// Advances every label reveal towards its target and writes the wrapper
+/// width + label alpha. Width is derived from the label's *live* font size,
+/// so the Fold's posture-change font rescale re-widens expanded labels
+/// automatically. Node writes are skipped when the width is unchanged to
+/// avoid dirtying UI layout every frame.
+pub(super) fn animate_tab_expansion(
+    time: Res<Time>,
+    settings: Option<Res<SettingsResource>>,
+    mut wraps: Query<(&mut TabLabelWrap, &mut Node, &Children)>,
+    mut labels: Query<(&TextFont, &mut TextColor), With<TabLabel>>,
+) {
+    let reduce_motion = settings.as_deref().is_some_and(|s| s.0.reduce_motion_mode);
+    let dt = time.delta_secs();
+    for (mut wrap, mut node, children) in &mut wraps {
+        for child in children.iter() {
+            let Ok((font, mut text_color)) = labels.get_mut(child) else {
+                continue;
+            };
+            let next = step_expansion(wrap.progress, wrap.target, dt, reduce_motion);
+            let width = expansion_width(wrap.label_chars, font.font_size, next);
+            // `!=` (not epsilon) is deliberate: last_width starts NaN so the
+            // first frame always writes, and equal floats mean a true no-op.
+            if next != wrap.progress || width != wrap.last_width {
+                wrap.progress = next;
+                wrap.last_width = width;
+                node.width = Val::Px(width);
+                text_color.0.set_alpha(next);
+            }
+            break;
+        }
+    }
+}
+
+/// Idle / hover / pressed feedback for glass tab buttons — the glass
+/// counterpart of `paint_action_buttons`, using each button's own colour
+/// triple so the accent-filled active pill and the transparent icon
+/// buttons share one system.
+pub(super) fn paint_glass_tab_buttons(
+    mut buttons: Query<(&Interaction, &GlassTabButton, &mut BackgroundColor), Changed<Interaction>>,
+) {
+    for (interaction, style, mut bg) in &mut buttons {
+        bg.0 = match interaction {
+            Interaction::Pressed => style.pressed,
+            Interaction::Hovered => style.hovered,
+            Interaction::None => style.idle,
+        };
+    }
+}