Fix nine-slice seams and inherited alpha

lib/runtime/rendering/render_tree_controller.dart

@@ -327,6 +327,7 @@ class RenderTreeController {
       contentOffsetY: parentContentOffset.y,
     );
     component.setViewportCulled(_isCulledByParentListView(component, parent));
+    component.setInheritedAlpha(parent?.renderAlpha ?? 1);
     if (component.parent == target) {
       return;
     }

lib/runtime/rendering/runtime_component.dart

@@ -43,6 +43,7 @@ List<({Rect source, Rect destination})> runtimeNineSliceRects({
   double sliceTop = 0,
   double sliceRight = 0,
   double sliceBottom = 0,
+  double destinationOverlap = 0,
 }) {
   if (source.width <= 0 ||
       source.height <= 0 ||
@@ -93,7 +94,7 @@ List<({Rect source, Rect destination})> runtimeNineSliceRects({
         sourceXs[x + 1],
         sourceYs[y + 1],
       );
-      final destPart = Rect.fromLTRB(
+      final rawDestPart = Rect.fromLTRB(
         destXs[x],
         destYs[y],
         destXs[x + 1],
@@ -101,16 +102,41 @@ List<({Rect source, Rect destination})> runtimeNineSliceRects({
       );
       if (sourcePart.width <= 0 ||
           sourcePart.height <= 0 ||
-          destPart.width <= 0 ||
-          destPart.height <= 0) {
+          rawDestPart.width <= 0 ||
+          rawDestPart.height <= 0) {
         continue;
       }
+      final destPart = _overlapNineSliceDestinationRect(
+        rawDestPart,
+        x: x,
+        y: y,
+        bounds: destination,
+        overlap: destinationOverlap,
+      );
       parts.add((source: sourcePart, destination: destPart));
     }
   }
   return parts;
 }
 
+Rect _overlapNineSliceDestinationRect(
+  Rect rect, {
+  required int x,
+  required int y,
+  required Rect bounds,
+  required double overlap,
+}) {
+  if (overlap <= 0) {
+    return rect;
+  }
+  return Rect.fromLTRB(
+    x == 0 ? rect.left : math.max(bounds.left, rect.left - overlap),
+    y == 0 ? rect.top : math.max(bounds.top, rect.top - overlap),
+    x == 2 ? rect.right : math.min(bounds.right, rect.right + overlap),
+    y == 2 ? rect.bottom : math.min(bounds.bottom, rect.bottom + overlap),
+  );
+}
+
 class RuntimeComponent extends PositionComponent
     with HasVisibility, TapCallbacks {
   RuntimeComponent({
@@ -157,8 +183,11 @@ class RuntimeComponent extends PositionComponent
   double _parentContentOffsetY = 0;
   bool _viewportCulled = false;
   bool _pressed = false;
+  double _inheritedAlpha = 1;
 
-  double get renderAlpha => _runtimeAlpha ?? _node.alpha;
+  double get localAlpha => _runtimeAlpha ?? _node.alpha;
+
+  double get renderAlpha => (_inheritedAlpha * localAlpha).clamp(0.0, 1.0);
 
   void setRuntimeAlpha(double value) {
     final next = value.clamp(0, 1).toDouble();
@@ -166,7 +195,23 @@ class RuntimeComponent extends PositionComponent
       return;
     }
     _runtimeAlpha = next;
+    _refreshInheritedAlphaSubtree();
+  }
+
+  void setInheritedAlpha(double value) {
+    final next = value.clamp(0, 1).toDouble();
+    if (_inheritedAlpha == next) {
+      return;
+    }
+    _inheritedAlpha = next;
+    _refreshInheritedAlphaSubtree();
+  }
+
+  void _refreshInheritedAlphaSubtree() {
     _syncTextStyle(_node);
+    for (final child in children.whereType<RuntimeComponent>()) {
+      child.setInheritedAlpha(renderAlpha);
+    }
   }
 
   void setParentScroll({
@@ -194,6 +239,7 @@ class RuntimeComponent extends PositionComponent
     _syncImage(node);
     _syncSpine(node);
     _syncParticle(node);
+    _refreshInheritedAlphaSubtree();
   }
 
   bool containsVisualPoint(Vector2 point) {
@@ -584,6 +630,7 @@ class RuntimeComponent extends PositionComponent
     final parts = runtimeNineSliceRects(
       source: source,
       destination: destination,
+      destinationOverlap: 0.5,
       sliceLeft: _node.sliceLeft ?? 0,
       sliceTop: _node.sliceTop ?? 0,
       sliceRight: _node.sliceRight ?? 0,
@@ -1174,6 +1221,7 @@ class RuntimeComponent extends PositionComponent
     _releaseLoadedSpine();
     _releaseParticle();
     _runtimeAlpha = null;
+    _inheritedAlpha = 1;
     _pressed = false;
     _textComponent = null;
     super.onRemove();