fix(scene): leaf widgets emit their own all_rounded_quads in paint_self (Phase 3)

paint_self reconstructed a widget's rounded background from color/corners, which missed widgets that override all_rounded_quads with custom geometry (e.g. Graph, whose nodes and edges are emitted there). Now: a leaf (no children) emits its own all_rounded_quads directly — capturing custom geometry and reducing to the plain rounded background for ordinary leaves, with no recursion since there are no children; a container still reconstructs its own plate (bevel/border/rounded) so the walk owns child recursion. Fixes missing graph edges under CCE_PAINT_WALK; 126 tests.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

 src/widget/mod.rs | 37 ++++++++++++++++++++++++-------------
 1 file changed, 24 insertions(+), 13 deletions(-)

diff --git a/src/widget/mod.rs b/src/widget/mod.rs
index 2cf71cc..91c8741 100644
--- a/src/widget/mod.rs
+++ b/src/widget/mod.rs
@@ -483,19 +483,30 @@ pub trait Element {
         let (x, y, w, h) = self.rect();
         let rect = Rect { x, y, width: w, height: h };
         let color = self.color();
-        let cr = self.corner_radii();
-        let radii = (cr.top_left, cr.top_right, cr.bottom_right, cr.bottom_left);
-
-        // The widget's own plate — mirrors `push_widget_vertices`: a beveled plate, or a bordered
-        // plate, or (the simple case) a rounded background from `rounded_corners`.
-        if let Some(depth) = self.plate_bevel() {
-            ctx.bevel(rect, radii, color, depth);
-        } else if let Some((border_color, thickness)) = self.solid_border() {
-            ctx.border(rect, radii, color, border_color, thickness);
-        } else if color[3].abs() > 0.001 {
-            let (r1, r2, r3, r4) = self.rounded_corners();
-            if r1 || r2 || r3 || r4 {
-                ctx.rounded_rect(rect, self.corner_radius(), (r1, r2, r3, r4), color);
+
+        if self.children(ui).is_empty() {
+            // Leaf: emit its own rounded quads directly. For an ordinary widget this is just the
+            // rounded background; for widgets that override `all_rounded_quads` with custom
+            // geometry (e.g. Graph's nodes and edges) it captures that too. No recursion happens
+            // because there are no children.
+            for (qx, qy, qw, qh, r, c, corners) in self.all_rounded_quads(ui) {
+                ctx.rounded_rect(Rect { x: qx, y: qy, width: qw, height: qh }, r, corners, c);
+            }
+        } else {
+            // Container: reconstruct its own plate (bevel / border / rounded background) — mirrors
+            // `push_widget_vertices`. Its children are drawn by the paint walk, so we must NOT call
+            // `all_rounded_quads` here (that would recurse and double-draw them).
+            let cr = self.corner_radii();
+            let radii = (cr.top_left, cr.top_right, cr.bottom_right, cr.bottom_left);
+            if let Some(depth) = self.plate_bevel() {
+                ctx.bevel(rect, radii, color, depth);
+            } else if let Some((border_color, thickness)) = self.solid_border() {
+                ctx.border(rect, radii, color, border_color, thickness);
+            } else if color[3].abs() > 0.001 {
+                let (r1, r2, r3, r4) = self.rounded_corners();
+                if r1 || r2 || r3 || r4 {
+                    ctx.rounded_rect(rect, self.corner_radius(), (r1, r2, r3, r4), color);
+                }
             }
         }