Fix runtime color alpha composition

lib/runtime/rendering/runtime_component.dart

@@ -12,6 +12,11 @@ import '../models/runtime_node.dart';
 import '../protocol/runtime_protocol.dart';
 import '../resources/game_resource_manager.dart';
 
+@visibleForTesting
+Color composeRuntimeColorAlpha(Color color, double alpha) {
+  return color.withValues(alpha: color.a * alpha.clamp(0.0, 1.0));
+}
+
 class RuntimeComponent extends PositionComponent
     with HasVisibility, TapCallbacks {
   RuntimeComponent({
@@ -156,7 +161,10 @@ class RuntimeComponent extends PositionComponent
     }
 
     final paint = Paint()
-      ..color = (_node.color ?? _defaultColor()).withValues(alpha: renderAlpha);
+      ..color = composeRuntimeColorAlpha(
+        _node.color ?? _defaultColor(),
+        renderAlpha,
+      );
 
     canvas.save();
     canvas.transform(Float64List.fromList(transform.transformMatrix.storage));
@@ -177,7 +185,10 @@ class RuntimeComponent extends PositionComponent
   @override
   void render(Canvas canvas) {
     final paint = Paint()
-      ..color = (_node.color ?? _defaultColor()).withValues(alpha: renderAlpha);
+      ..color = composeRuntimeColorAlpha(
+        _node.color ?? _defaultColor(),
+        renderAlpha,
+      );
 
     switch (_node.type) {
       case RuntimeNodeType.circle:
@@ -224,7 +235,7 @@ class RuntimeComponent extends PositionComponent
     final rect = Rect.fromLTWH(0, 0, size.x, size.y);
     final radius = Radius.circular(_node.radius ?? 4);
     final backgroundPaint = Paint()
-      ..color = const Color(0x33475569).withValues(alpha: renderAlpha);
+      ..color = composeRuntimeColorAlpha(const Color(0x33475569), renderAlpha);
     canvas.drawRRect(RRect.fromRectAndRadius(rect, radius), backgroundPaint);
 
     final value = _node.value ?? 0;
@@ -327,11 +338,15 @@ class RuntimeComponent extends PositionComponent
     final contentRect = _listViewContentRect();
     final scrollbars = _listViewScrollbarVisibility();
     final trackPaint = Paint()
-      ..color = (_node.scrollbarTrackColor ?? const Color(0x33475569))
-          .withValues(alpha: renderAlpha);
+      ..color = composeRuntimeColorAlpha(
+        _node.scrollbarTrackColor ?? const Color(0x33475569),
+        renderAlpha,
+      );
     final thumbPaint = Paint()
-      ..color = (_node.scrollbarThumbColor ?? const Color(0xaa94a3b8))
-          .withValues(alpha: renderAlpha);
+      ..color = composeRuntimeColorAlpha(
+        _node.scrollbarThumbColor ?? const Color(0xaa94a3b8),
+        renderAlpha,
+      );
 
     final contentHeight = _node.contentHeight;
     if (scrollbars.vertical &&
@@ -415,7 +430,7 @@ class RuntimeComponent extends PositionComponent
         image,
         Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble()),
         rect,
-        Paint()..color = Colors.white.withValues(alpha: renderAlpha),
+        Paint()..color = composeRuntimeColorAlpha(Colors.white, renderAlpha),
       );
       return;
     }
@@ -948,7 +963,7 @@ class RuntimeComponent extends PositionComponent
     final text = label ?? '';
     final color = _textColor(node);
     final style = TextStyle(
-      color: color.withValues(alpha: renderAlpha),
+      color: composeRuntimeColorAlpha(color, renderAlpha),
       fontSize: node.fontSize ?? 18,
       fontWeight: node.type == RuntimeNodeType.button
           ? FontWeight.w600