Use GetEndTime() to get storyboard endtime

osu.Game/Storyboards/IStoryboardElement.cs

@@ -14,4 +14,17 @@ namespace osu.Game.Storyboards
 
         Drawable CreateDrawable();
     }
+
+    public static class StoryboardElementExtensions
+    {
+        /// <summary>
+        /// Returns the end time of this object.
+        /// </summary>
+        /// <remarks>
+        /// This returns the <see cref="IStoryboardElementHasDuration.EndTime"/> where available, falling back to <see cref="IStoryboardElement.StartTime"/> otherwise.
+        /// </remarks>
+        /// <param name="storyboardElement">The object.</param>
+        /// <returns>The end time of this object.</returns>
+        public static double GetEndTime(this IStoryboardElement storyboardElement) => (storyboardElement as IStoryboardElementHasDuration)?.EndTime ?? storyboardElement.StartTime;
+    }
 }

osu.Game/Storyboards/IStoryboardElementHasDuration.cs

@@ -3,6 +3,9 @@
 
 namespace osu.Game.Storyboards
 {
+    /// <summary>
+    /// A StoryboardElement that ends at a different time than its start time.
+    /// </summary>
     public interface IStoryboardElementHasDuration
     {
         double EndTime { get; }

osu.Game/Storyboards/Storyboard.cs

@@ -44,7 +44,7 @@ namespace osu.Game.Storyboards
         /// This iterates all elements and as such should be used sparingly or stored locally.
         /// Videos and samples return StartTime as their EndTIme.
         /// </remarks>
-        public double? LatestEventTime => Layers.SelectMany(l => l.Elements.OfType<IStoryboardElementHasDuration>()).OrderByDescending(e => e.EndTime).FirstOrDefault()?.EndTime;
+        public double? LatestEventTime => Layers.SelectMany(l => l.Elements).OrderBy(e => e.GetEndTime()).LastOrDefault()?.GetEndTime();
 
         /// <summary>
         /// Depth of the currently front-most storyboard layer, excluding the overlay layer.