test: Add comprehensive tests for VisualState transition selection

[Copilot]

Overview

This PR adds comprehensive unit tests for the VisualState transition selection logic as requested in issue #17436. The tests validate the FindTransition method in VisualStateGroup.cs which determines which transition to apply when changing between visual states.

Changes

Added 9 new test methods to src/Uno.UI.Tests/Windows_UI_Xaml/VisualStateManagerTests/Given_VisualStateManager.cs that comprehensively cover the transition selection algorithm.

Transition Selection Algorithm

The FindTransition method implements a priority-based selection algorithm:

1. Perfect Match (highest priority): Transition with both From and To matching the state change
2. From Match: Transition with only From matching (and To is null/empty)
3. To Match (lowest priority): Transition with only To matching (and From is null/empty)
4. No Match: Returns null if no transition matches

When multiple transitions match at the same priority level, the first declared transition wins.

Test Coverage

Basic Matching Scenarios

• ✅ When_Transition_With_Both_From_And_To_Matches - Validates perfect match selection
• ✅ When_Transition_With_Only_From_Matches - Validates From-only matching
• ✅ When_Transition_With_Only_To_Matches - Validates To-only matching

Precedence Rules

• ✅ When_Multiple_Transitions_PerfectMatch_Takes_Precedence - Validates that perfect match has highest priority
• ✅ When_Multiple_Transitions_From_Takes_Precedence_Over_To - Validates that From-only beats To-only
• ✅ When_First_Matching_Transition_Is_Selected - Validates first-declared wins when multiple match

Edge Cases

• ✅ When_No_Transition_Matches - Validates behavior when no transition matches
• ✅ When_Transition_From_Initial_To_State1 - Validates transitions from initial state
• ✅ When_UseTransitions_False_No_Transition_Applied - Validates the useTransitions flag

Implementation Details

All tests follow the existing patterns in the file:

• Use existing helper methods (Setup(), Transition(), AnimateTag())
• Validate behavior by tracking Control.Tag property changes through RegisterPropertyChangedCallback
• Use SequenceEqual to verify the exact sequence of state changes
• Maintain consistency with existing test style and conventions

Example Test

[TestMethod]
public void When_Multiple_Transitions_PerfectMatch_Takes_Precedence()
{
    var (control, group) = Setup();
    // Add transitions in specific order to test precedence
    var transitionFrom = Transition(control, from: 1, to: null, frames: 10);
    var transitionTo = Transition(control, from: null, to: 2, frames: 20);
    var transitionPerfect = Transition(control, from: 1, to: 2, frames: 0);

    group.Transitions.Add(transitionFrom);
    group.Transitions.Add(transitionTo);
    group.Transitions.Add(transitionPerfect);

    var tags = new List<string> { (string)control.Tag };
    control.RegisterPropertyChangedCallback(Control.TagProperty, (_, __) => tags.Add((string)control.Tag));

    VisualStateManager.GoToState(control, "state1", true);
    VisualStateManager.GoToState(control, "state2", true);

    // Perfect match (both from and to) should be selected
    Assert.IsTrue(new[] { "initial", "state1", "transition_from_state1_to_state2_frame_0", "state2" }.SequenceEqual(tags));
}

Related

• Closes fix: Fix Visual transition not applied properly #17436
• Related to PR fix: Fix Visual transition not applied properly #17436 (original implementation)

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add tests for VIsualState transition selection</issue_title>
<issue_description>### Current behavior

No response

Expected behavior

Add tests for #17436

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

No response

Workaround

No response

Works on UWP/WinUI

None

Environment

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NuGet package version(s)

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Affected platforms

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IDE

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IDE version

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Relevant plugins

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Anything else we need to know?

No response</issue_description>

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

Fixes #17564

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