fix: Storyboard Completed event not firing for dependent animations without EnableDependentAnimation

[Copilot]

Storyboard animations targeting dependent properties (Height, Width, Margin, etc.) without EnableDependentAnimation=true would never fire their Completed event, breaking animation chains and causing applications to hang waiting for completion.

Root Cause

When a dependent animation was skipped, the Play() method returned early without triggering any completion logic:

if (!EnableDependentAnimation && _owner.GetIsDependantAnimation())
{
    return;  // Animation silently dies here
}

Changes

• Timeline.animation.cs, ColorAnimationUsingKeyFrames.cs, DoubleAnimationUsingKeyFrames.cs: Schedule completion via dispatcher when dependent animations are skipped to maintain WinUI3 parity
• Given_Storyboard.cs: Add runtime tests validating Completed fires for skipped dependent animations and doesn't fire when explicitly stopped

Behavior

var animation = new DoubleAnimation 
{ 
    To = 200, 
    EnableDependentAnimation = false  // Not set to true
};
Storyboard.SetTargetProperty(animation, "Height");  // Dependent property

var storyboard = new Storyboard();
storyboard.Completed += OnCompleted;  // Now fires immediately (previously: never)
storyboard.Begin();

The animation still doesn't run (existing behavior), but completion handlers now execute as in WinUI3.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Skia][Animation] Issue with Storyboard Animation's Completed Event Not Triggering</issue_title>
<issue_description>### Current behavior

I am writing to report an issue I've encountered while using Storyboard to play animations. The problem arises when certain layout-related properties are involved in my animation, and the EnableDependentAnimation attribute is not set to true. In this case, the Completed event of the Storyboard animation never triggers.

Expected behavior

The behavior should remain consistent with WinUI3, allowing the Completed event of the Storyboard animation to trigger.

How to reproduce it (as minimally and precisely as possible)

Steps to Reproduce:

1. Set up the XAML interface and animation resources as follows:

  <Page.Resources>
    <Storyboard x:Key="FooBorderStoryboard">
      <DoubleAnimation
        Storyboard.TargetName="FooBorder"
        Storyboard.TargetProperty="Width"
        To="1000"
        Duration="0:0:3"/>
    </Storyboard>
  </Page.Resources>
  <Grid>
    <Border x:Name="FooBorder" Background="Blue" Width="500" Height="500" HorizontalAlignment="Left" VerticalAlignment="Top"/>
  </Grid>

2. Execute the animation through the code and listen for the Completed event:

    public MainPage()
    {
        this.InitializeComponent();
        Loaded += MainPage_Loaded;
    }

    private async void MainPage_Loaded(object sender, RoutedEventArgs e)
    {
        await Task.Delay(200);
        var storyboard = (Storyboard) Resources["FooBorderStoryboard"];
        storyboard.Completed += Storyboard_Completed;
        storyboard.Begin();
    }

    private void Storyboard_Completed(object? sender, object e)
    {
        Console.WriteLine("Storyboard_Completed");
    }

3. Run the project on both Desktop and WinUI platforms.

You will notice that on the Desktop platform, the Storyboard_Completed method is never entered. However, on the WinUI platform, this method is entered and output is displayed on the console.

You can find my demo project at the following link: https://github.com/lindexi/lindexi_gd/tree/9bd249807ed238ad6fb5b1cd2f706a2749f472e4/UnoDemo/LairhalawcarchemKacallligekay

Workaround

No response

Works on UWP/WinUI

Yes

Environment

No response

NuGet package version(s)

global.json

{
  // To update the version of Uno please update the version of the Uno.Sdk here. See https://aka.platform.uno/upgrade-uno-packages for more information.
  "msbuild-sdks":
   {
    "Uno.Sdk": "5.2.175"
  },
  "sdk": 
  {
    "allowPrerelease": false
  }
}

Affected platforms

No response

IDE

No response

IDE version

No response

Relevant plugins

No response

Anything else we need to know?

I can find the code in console after I add the builder.AddFilter("Microsoft.UI.Xaml.Media.Animation", LogLevel.Debug); log filter

dbug: Microsoft.UI.Xaml.Media.Animation.Timeline[0]
      This Dependent animation will not run, EnableDependentAnimation is set to false
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes [Skia][Animation] Issue with Storyboard Animation's Completed Event Not Triggering #17596

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