LineRendererConstantColorPropertyField.cs

using UnityEditor;
using UnityEngine;

namespace Kogane.Internal
{
    [InitializeOnLoad]
    internal static class LineRendererConstantColorPropertyField
    {
        static LineRendererConstantColorPropertyField()
        {
            _ = new EditorGUIUtility.PropertyCallbackScope( Callback );

            static void Callback
            (
                Rect               rect,
                SerializedProperty serializedProperty
            )
            {
                if ( serializedProperty.serializedObject.targetObject.GetType().Name != nameof( LineRenderer ) ) return;
                if ( serializedProperty.propertyPath != "m_Parameters.colorGradient" ) return;

                var color = serializedProperty.gradientValue.colorKeys[ 0 ].color;

                using var changeCheckScope = new EditorGUI.ChangeCheckScope();

                var newColor = EditorGUILayout.ColorField( "Constant Color", color );

                if ( !changeCheckScope.changed ) return;

                var newGradient = new Gradient();
                var newAlpha    = newColor.a;

                newGradient.SetKeys
                (
                    new GradientColorKey[] { new( newColor, 0 ), new( newColor, 1 ) },
                    new GradientAlphaKey[] { new( newAlpha, 0 ), new( newAlpha, 1 ) }
                );

                serializedProperty.gradientValue = newGradient;
            }
        }
    }
}