Description

This PR fixes an issue on Windows where setting LineHeight to values less than 1 had no visual effect on Labels. The problem was that WinUI TextBlock's default LineStackingStrategy is MaxHeight, which ignores LineHeight values that are smaller than the natural line height.

Problem

When using LineHeight multipliers < 1 on Windows Labels, the line spacing would not decrease as expected:

<Label Text="Test text with tight line spacing" 
       LineHeight="0.8" 
       FontSize="20" />

Before fix: LineHeight="0.8" had no visual effect - lines remained at default spacing
After fix: LineHeight="0.8" properly reduces line spacing

Root Cause

The WinUI TextBlock control has two relevant properties:

• LineHeight: Sets the desired line height
• LineStackingStrategy: Determines how the LineHeight value is applied

The default LineStackingStrategy.MaxHeight uses the larger of:

• The natural height needed for the text
• The specified LineHeight value

This means LineHeight values < 1 are effectively ignored since the natural height is typically larger.

Solution

Modified the UpdateLineHeight method in TextBlockExtensions.cs to set LineStackingStrategy to BlockLineHeight when a LineHeight is specified. BlockLineHeight always uses the exact LineHeight value, allowing values < 1 to take effect.

public static void UpdateLineHeight(this TextBlock platformControl, ILabel label)
{
    if (label.LineHeight >= 0)
    {
        platformControl.LineHeight = label.LineHeight * platformControl.FontSize;
        platformControl.LineStackingStrategy = LineStackingStrategy.BlockLineHeight; // NEW
    }
}

Additional Changes

• Enabled LineHeight tests for Windows platform (removed #if !WINDOWS conditionals)
• Added GetNativeLineHeight() and GetNativeCharacterSpacing() methods for Windows test infrastructure
• Added specific tests to verify LineStackingStrategy behavior

Testing

The fix ensures consistent LineHeight behavior across all platforms:

• LineHeight < 1: Now properly reduces line spacing on Windows
• LineHeight > 1: Continues to work as before
• LineHeight = -1 (default): Uses default behavior (MaxHeight strategy)
• Cross-platform consistency: Windows now matches iOS and Android behavior

Fixes #24520.

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