[libTxt] resolve null leading distribution in dart:ui.

[LongCatIsLooong]

Remove {StrutStyle, TextStyle}.has_leading_distribution_override from libTxt, and make {StrutStyle, TextStyle}.half_leading final (so they'll never default to ParagraphStyle.text_height_behavior during layout, instead they are resolved earlier than that: when the paragraph is being built).

SkParagraph relies on TextStyle.== to break TextStyles into Runs. If TextStyle.leadingDistribution may depend on ParagraphStyle then we won't be able to provide a reasonable TextStyle.== implementation. Plus it makes more sense to me to do the defaulting in dart:ui.

Related Issue: flutter/flutter#72521

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[jason-simmons]

Should leadingDistribution be taken out of the TextHeightBehavior object?

leadingDistribution is an attribute that can be applied to individual runs of text. But the other members of TextHeightBehavior can only apply to the whole paragraph.

[LongCatIsLooong]

In the framework TextStyle.leadingnDistribution can be null in which case it defers to ParagraphStyle.textHeightBehavior.leadingDistribution. You mean we should add it as a separate field to ParagraphStyle?

[LongCatIsLooong]

That does make more sense for fromEncoded and encode. But that would break some tests in the framework. Would it be possible to make these 2 methods private?

[jason-simmons]

Yes - I think it would be more consistent to make leadingDistribution a field in ParagraphStyle that can optionally be overridden in the TextStyle

AFAIK there isn't any need for TextHeightBehavior.encode and TextHeightBehavior.fromEncoded to be public.

Most other encode/decode methods for these objects are private (like _encodeParagraphStyle and _encodeTextStyle)

[LongCatIsLooong]

Looking at the framework ParagraphStyle code, moving leadingDistribution to ParagraphStyle is going to break it. Is there a process for making such changes? Or I can hide the ugliness by making these methods private at the cost of testability.

[LongCatIsLooong]

@jason-simmons #25627 includes the skparagraph half leading commit. Updated the TxtToSkia method.