Fix ApplyToDerivedTypes not working for implicit styles (#9648)

[StephaneDelcroix]

This fixes issue #9648 where ApplyToDerivedTypes was not working correctly for implicit styles. Only the most specific style was being applied, and base class styles were being ignored.

Changes:

• Fixed MergedStyle.OnImplicitStyleChanged() to collect and merge all applicable implicit styles from the type hierarchy
• Added unit tests (Maui9648) that verify the fix works correctly
• Added performance benchmarks to measure impact

Test Results:

• New tests: 6/6 PASS (all inflators: Runtime, XamlC, SourceGen)
• Existing tests: 7,159/7,159 PASS (0 regressions)

Performance Impact (measured with BenchmarkDotNet):

• Single implicit style: 1.68x baseline (minimal overhead, optimized)
• Two implicit styles: 3.35x baseline (+11.5 μs per element)
• Three implicit styles: 5.33x baseline (+21 μs per element)
• Overhead is one-time cost at element creation, not per-render
• Real-world impact: Imperceptible for typical usage (< 1-2ms per page)

Fixes #9648

Note

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It would be very helpful if you could test the resulting artifacts from this PR and let us know in a comment if this change resolves your issue. Thank you!

Description of Change

Issues Fixed

Fixes #9648

[Copilot]

Pull Request Overview

This PR fixes issue #9648 where ApplyToDerivedTypes was not working correctly for implicit styles. Previously, only the most specific implicit style was applied to elements, ignoring styles from base types in the hierarchy. The fix modifies MergedStyle.OnImplicitStyleChanged() to collect all applicable implicit styles from the type hierarchy and merge them using style chaining.

Key Changes:

• Modified the implicit style resolution algorithm to collect all applicable styles instead of returning early after finding the first one
• Implemented style merging by creating a BasedOn chain from base to derived types
• Added optimization for single-style case to avoid overhead
• Added comprehensive unit tests and performance benchmarks

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Controls/src/Core/MergedStyle.cs	Core fix - collects and merges multiple implicit styles using BasedOn chaining
src/Controls/tests/Xaml.UnitTests/Issues/Maui9648.xaml	Test XAML with three-level style hierarchy (InputView → Entry → CustomEntry)
src/Controls/tests/Xaml.UnitTests/Issues/Maui9648.xaml.cs	Unit tests verifying style inheritance works correctly
src/Controls/tests/Xaml.Benchmarks/ImplicitStylesBenchmark.cs	Performance benchmarks measuring overhead of style merging

[Copilot]

Behaviors and triggers should not be copied into the wrapper style. Since the wrapper style has BasedOn = mergedStyle (line 186), behaviors and triggers from base styles will already be applied through the BasedOn chain when the style is applied (see Style.ApplyCore at line 166 which applies the BasedOn style first). Copying them here will cause behaviors and triggers to be attached multiple times to the same element, leading to duplicate behavior execution and potential state management issues. Remove the behavior and trigger copying logic (lines 197-213).

Suggested change

	// Copy behaviors if any exist
	if (currentStyle.Behaviors.Count > 0)
	{
	foreach (var behavior in currentStyle.Behaviors)
	{
	wrapperStyle.Behaviors.Add(behavior);
	}
	}
	// Copy triggers if any exist
	if (currentStyle.Triggers.Count > 0)
	{
	foreach (var trigger in currentStyle.Triggers)
	{
	wrapperStyle.Triggers.Add(trigger);
	}
	}

[StephaneDelcroix]

/azp run MAUI-UITests-public

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