feat: cce-bevel transparency dialog

An Opacity button beside Save/Reset toggles a small dialog — a raised
plate floated over the cutaway, its own alpha pinned readable — holding
one band slider that drives the window plate's alpha live (the popup
becomes its own glass sample). Escape and press-outside dismiss, ahead
of the Escape-exits default; presses on the toggle button fall through
so it can close the dialog itself.

Co-Authored-By: Claude Fable 5 <[email protected]>

 src/bin/cce-bevel.rs | 111 ++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 100 insertions(+), 11 deletions(-)

diff --git a/src/bin/cce-bevel.rs b/src/bin/cce-bevel.rs
index 8516a40..2362a42 100644
--- a/src/bin/cce-bevel.rs
+++ b/src/bin/cce-bevel.rs
@@ -149,6 +149,14 @@ struct BevelPopup {
     width_slider: Adapted<Slider>,
     save_button: Adapted<Button>,
     reset_button: Adapted<Button>,
+    /// The transparency dialog: a raised plate floated over the popup with
+    /// one slider driving the window plate's alpha, toggled by its button.
+    opacity_button: Adapted<Button>,
+    opacity_slider: Adapted<Slider>,
+    opacity_open: bool,
+    plate_opacity: f32,
+    dialog_rect: Rect,
+    opacity_btn_rect: Rect,
     /// Status line under the buttons: what the last save/reset did.
     status: String,
     ui_context: cce_ui::context::UiContext,
@@ -309,8 +317,10 @@ fn draw_section(pc: &mut PaintCtx, rect: Rect, profile: &ProfileKnobs, has_floor
 }
 
 impl BevelPopup {
-    fn root_ids(&self) -> [WidgetId; 11] {
+    fn root_ids(&self) -> [WidgetId; 13] {
         [
+            self.opacity_slider.id(),
+            self.opacity_button.id(),
             self.profile_dropdown.id(),
             self.wall.shoulder.id(),
             self.wall.base.id(),
@@ -325,8 +335,10 @@ impl BevelPopup {
         ]
     }
 
-    fn roots(&mut self) -> [*mut (dyn WidgetHost + 'static); 11] {
+    fn roots(&mut self) -> [*mut (dyn WidgetHost + 'static); 13] {
         [
+            self.opacity_slider.as_ptr_mut(),
+            self.opacity_button.as_ptr_mut(),
             self.profile_dropdown.as_ptr_mut(),
             self.wall.shoulder.as_ptr_mut(),
             self.wall.base.as_ptr_mut(),
@@ -391,6 +403,15 @@ impl BevelPopup {
             self.reset_live();
             self.needs_rebuild = true;
         }
+        if self.opacity_button.take_click() {
+            self.opacity_open = !self.opacity_open;
+            self.needs_rebuild = true;
+        }
+        if self.opacity_slider.take_change() {
+            self.plate_opacity = self.opacity_slider.inner().value();
+            println!("opacity {:.2}", self.plate_opacity);
+            self.needs_rebuild = true;
+        }
     }
 
     /// Persist the current material to the shared config
@@ -496,6 +517,18 @@ impl Application for BevelPopup {
                 .with_band(true),
             save_button: Button::new(0.0, 0.0, 0.0, 0.0).with_label("Save"),
             reset_button: Button::new(0.0, 0.0, 0.0, 0.0).with_label("Reset"),
+            opacity_button: Button::new(0.0, 0.0, 0.0, 0.0).with_label("Opacity"),
+            opacity_slider: Slider::new()
+                .with_label("Opacity")
+                .with_value(cce_ui::color::active_backplate_opacity().clamp(0.0, 1.0))
+                .with_readout(true)
+                .with_decimals(2)
+                .with_scroll(true)
+                .with_band(true),
+            opacity_open: false,
+            plate_opacity: cce_ui::color::active_backplate_opacity(),
+            dialog_rect: Rect::ZERO,
+            opacity_btn_rect: Rect::ZERO,
             status: "Edits apply live; Save writes config.kdl.".to_string(),
             ui_context: cce_ui::context::UiContext::new(),
             width: 520,
@@ -611,9 +644,35 @@ impl Application for BevelPopup {
             y += knob_h + gap;
             self.save_button.set_rect(x, y, 96.0, button_h);
             self.reset_button.set_rect(x + 96.0 + 12.0, y, 96.0, button_h);
+            self.opacity_btn_rect = Rect { x: x + 2.0 * (96.0 + 12.0), y, width: 96.0, height: button_h };
+            self.opacity_button.set_rect(
+                self.opacity_btn_rect.x,
+                self.opacity_btn_rect.y,
+                self.opacity_btn_rect.width,
+                self.opacity_btn_rect.height,
+            );
             y += button_h + 8.0;
             self.status_pos = (x, y);
 
+            // The transparency dialog floats centered over the cutaway; its
+            // slider parks off-screen while closed.
+            self.dialog_rect = Rect {
+                x: x + (w - 300.0).max(0.0) / 2.0,
+                y: self.cut_rect.y + (self.cut_rect.height - 92.0).max(0.0) / 2.0,
+                width: 300.0f32.min(w),
+                height: 92.0,
+            };
+            if self.opacity_open {
+                self.opacity_slider.set_rect(
+                    self.dialog_rect.x + 18.0,
+                    self.dialog_rect.y + (92.0 - knob_h) / 2.0 + 4.0,
+                    self.dialog_rect.width - 36.0,
+                    knob_h,
+                );
+            } else {
+                self.opacity_slider.set_rect(-1000.0, -1000.0, 0.0, 0.0);
+            }
+
             self.needs_rebuild = false;
             self.ui_context.rebuild_spatial_grid();
         }
@@ -630,9 +689,7 @@ impl Application for BevelPopup {
         // previews the edge profile, and the wells/buttons preview the wall
         // profile — the popup is its own material sample.
         let mut plate = cce_ui::color::page_low_color();
-        if plate[3] > 0.001 {
-            plate[3] = cce_ui::color::active_backplate_opacity();
-        }
+        plate[3] = self.plate_opacity;
         let radius = cce_ui::colors::backplate_corner_radius();
         let bevel = cce_ui::layout::bevel_width();
         pc.plate(
@@ -669,6 +726,17 @@ impl Application for BevelPopup {
         cce_ui::scene::painter::paint_root_into(&self.ui_context, &self.profile_dropdown, &mut pc);
         cce_ui::scene::painter::paint_root_into(&self.ui_context, &self.save_button, &mut pc);
         cce_ui::scene::painter::paint_root_into(&self.ui_context, &self.reset_button, &mut pc);
+        cce_ui::scene::painter::paint_root_into(&self.ui_context, &self.opacity_button, &mut pc);
+
+        // The transparency dialog: a raised plate floated over the cutaway,
+        // its own alpha pinned readable so the slider stays usable at glassy
+        // window settings.
+        if self.opacity_open {
+            let mut dc = cce_ui::color::page_low_color();
+            dc[3] = 0.96;
+            pc.plate(self.dialog_rect, (10.0, 10.0, 10.0, 10.0), dc, bevel * 0.75);
+            cce_ui::scene::painter::paint_root_into(&self.ui_context, &self.opacity_slider, &mut pc);
+        }
 
         // The selector's popover, drawn into the frame on top of everything
         // below it (its labels carry the popover rect as bounds).
@@ -758,6 +826,21 @@ impl Application for BevelPopup {
             self.needs_rebuild = true;
             return None;
         }
+        // A press outside the open transparency dialog dismisses it (presses
+        // on its toggle button fall through — the button click handles it).
+        if self.opacity_open && state == ElementState::Pressed {
+            let d = self.dialog_rect;
+            let b = self.opacity_btn_rect;
+            let inside = |r: Rect| {
+                pos.x >= r.x && pos.x <= r.x + r.width && pos.y >= r.y && pos.y <= r.y + r.height
+            };
+            if !inside(d) && !inside(b) {
+                self.opacity_open = false;
+                *needs_rebuild = true;
+                self.needs_rebuild = true;
+                return None;
+            }
+        }
         let ev = Event::MouseButton {
             button,
             state,
@@ -813,12 +896,18 @@ impl Application for BevelPopup {
     ) -> Option<Self::Message> {
         use cce_ui::widget::{Key, NamedKey};
         if event.state == ElementState::Pressed {
-            // Escape exits — unless the context menu is up (the toolkit-wide
-            // Escape-dismiss should win the first press).
-            if event.logical_key == Key::Named(NamedKey::Escape)
-                && !self.ui_context.is_context_menu_visible()
-            {
-                return Some(BevelMsg::Exit);
+            // Escape closes the transparency dialog first, then the context
+            // menu (toolkit-wide default), and only then exits.
+            if event.logical_key == Key::Named(NamedKey::Escape) {
+                if self.opacity_open {
+                    self.opacity_open = false;
+                    *needs_rebuild = true;
+                    self.needs_rebuild = true;
+                    return None;
+                }
+                if !self.ui_context.is_context_menu_visible() {
+                    return Some(BevelMsg::Exit);
+                }
             }
             if event.ctrl {
                 if let Key::Character(ref c) = event.logical_key {