Improve Dropdown Control- #4102

Merges #4102

Source/Engine/Core/Math/Color.cs

@@ -112,6 +112,16 @@ namespace FlaxEngine
         /// </summary>
         public float ValuesSum => R + G + B + A;
 
+        /// <summary>
+        /// Returns true if color is fully transparent (all components are equal zero).
+        /// </summary>
+        public bool IsTransparent => Mathf.IsZero(R + G + B + A);
+
+        /// <summary>
+        /// Returns true if color has opacity channel in use (different from 1).
+        /// </summary>
+        public bool HasOpacity => !Mathf.IsOne(A);
+
         /// <summary>
         /// Constructs a new Color with given r,g,b,a component.
         /// </summary>

Source/Engine/UI/GUI/Common/Dropdown.cs

@@ -595,7 +595,7 @@ namespace FlaxEngine.GUI
                 Size = new Float2(size.X - margin, size.Y),
                 Font = Font,
                 TextColor = TextColor * 0.9f,
-                TextColorHighlighted = BackgroundColorSelected.Brightness < 0.05f ? Color.Lerp(TextColorHighlighted, Color.White, 0.3f) : TextColorHighlighted,
+                TextColorHighlighted = (Mathf.IsZero(BackgroundColorSelected.Brightness) || BackgroundColorSelected.IsTransparent) ? Color.Lerp(TextColorHighlighted, Color.White, 0.3f) : TextColorHighlighted,
                 HorizontalAlignment = HorizontalAlignment,
                 VerticalAlignment = VerticalAlignment,
                 Text = _items[i],