[Feature] Move LetterSpacing to TextElement as inherited attached property

[jsuarezruiz]

This PR relocates the LetterSpacing property from TextBlock to TextElement as an inherited attached property, making it available across all templated controls (Button, CheckBox, Label, ToolTip, etc.). This change aligns LetterSpacing with the existing architecture used by FontFamily, FontSize, FontWeight, and other text properties.

// TextElement (new)
public static readonly AttachedProperty<double> LetterSpacingProperty;
public static double GetLetterSpacing(Control control);
public static void SetLetterSpacing(Control control, double value);

// TemplatedControl (new)
public static readonly StyledProperty<double> LetterSpacingProperty;
public double LetterSpacing { get; set; }

// TextBlock (unchanged, now uses AddOwner internally)
public static readonly StyledProperty<double> LetterSpacingProperty;
public double LetterSpacing { get; set; }

Usage Examples:

<Button Content="Click Me" LetterSpacing="2" />
<CheckBox Content="Accept Terms" LetterSpacing="0.5" />

Screenshots with some examples:

Checkbox

RadioButton

Menu

ComboBox

Button

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None. All existing code continues to work without modification.

Obsoletions / Deprecations

Fixed issues

Fixes #20119

[MrJul]

There is a breaking change: TextBlock.LetterSpacing is now an AttachedProperty instead of a StyledProperty. This is not binary compatible, and would also be a source break to existing usages of TextBlock.TextSpacing as an attached property.

We can totally accept this minor change for v12, but it should not be backported to v11.

[cla-avalonia]

@jsuarezruiz,

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[MrJul]

Really nitpicking here, but seeing the difference between the different values would be better without the extra spaces in the string.

[jsuarezruiz]

Yeah, you are right. Removed the spaces, now it's obvious that the LetterSpacing property is what creates the spacing between characters.

[MrJul]

This is an inherited property, we don't usually set it explicitly in templates (see also FontFamily, FontWeight, etc.)

Applicable to all modified templates.

[robloo]

Yea, the WPF convention of letting all font/text properties be properly inherited needs to apply. For some reason in WinUI they started setting everything directly in Fluent. That was an issue and we had to undo a lot of it here and in FluentAvalonia. Definately don't want to take a step back.

[jsuarezruiz]

Good point. Removed all explicit TextElement.LetterSpacing="{TemplateBinding LetterSpacing}" bindings from templates. The property now inherits automatically through the visual tree, just like FontFamily, FontWeight, FontSize, and other inherited text properties.

[MrJul]

I'm normally always advocating for more tests, but this time having those tests duplicated in Button, CheckBox, Label, and RadioButton doesn't provide much value, only added maintenance :)

We already have tests verifying that inherited styled properties are working as expected in the general case. The LetterSpacing-specific tests can be added only once.

[MrJul]

You'll need to run ./build.sh ValidateApiDiff to update the API suppression files.

[robloo]

It's awesome to see you on the Avalonia team now! I remember talking about your drawn controls for MAUI some time ago. Was some interesting work there and in MAUI itself. Anyway, welcome to Avalonia!

[jsuarezruiz]

It's awesome to see you on the Avalonia team now! I remember talking about your drawn controls for MAUI some time ago. Was some interesting work there and in MAUI itself. Anyway, welcome to Avalonia!

It's awesome to see you on the Avalonia team now! I remember talking about your drawn controls for MAUI some time ago. Was some interesting work there and in MAUI itself. Anyway, welcome to Avalonia!

Thanks so much! I appreciate it.
Yes, I remember those drawn controls and all the MAUI work were a fun challenge.
Excited to join Avalonia and help push things forward!