style: Resolve <number> to <length> in ToResolvedValue for line-height

This makes the serialization of the 'font' shorthand on computed style return the line-height
as an absolute length rather than a number (font-size multiplier), which is consistent with
what the line-height longhand already returns, and with other browsers.

(See also https://github.com/w3c/csswg-drafts/issues/8385.)

Differential Revision: https://phabricator.services.mozilla.com/D168542

components/style/values/computed/text.rs

@@ -11,6 +11,7 @@ use crate::values::computed::{Context, NonNegativeLength, NonNegativeNumber, ToC
 use crate::values::generics::text::InitialLetter as GenericInitialLetter;
 use crate::values::generics::text::LineHeight as GenericLineHeight;
 use crate::values::generics::text::{GenericTextDecorationLength, Spacing};
+use crate::values::resolved::{Context as ResolvedContext, ToResolvedValue};
 use crate::values::specified::text::{self as specified, TextOverflowSide};
 use crate::values::specified::text::{TextEmphasisFillMode, TextEmphasisShapeKeyword};
 use crate::values::{CSSFloat, CSSInteger};
@@ -113,6 +114,25 @@ impl ToComputedValue for specified::WordSpacing {
 /// A computed value for the `line-height` property.
 pub type LineHeight = GenericLineHeight<NonNegativeNumber, NonNegativeLength>;
 
+impl ToResolvedValue for LineHeight {
+    type ResolvedValue = Self;
+
+    fn to_resolved_value(self, context: &ResolvedContext) -> Self::ResolvedValue {
+        // Resolve <number> to an absolute <length> based on font size.
+        if let LineHeight::Number(num) = &self {
+            let size = context.style.get_font().clone_font_size().computed_size();
+            LineHeight::Length(NonNegativeLength::new(size.px() * num.0))
+        } else {
+            self
+        }
+    }
+
+    #[inline]
+    fn from_resolved_value(value: Self::ResolvedValue) -> Self {
+        value
+    }
+}
+
 impl WordSpacing {
     /// Return the `normal` computed value, which is just zero.
     #[inline]

components/style/values/generics/text.rs

@@ -92,7 +92,6 @@ fn line_height_moz_block_height_enabled(context: &ParserContext) -> bool {
     ToAnimatedValue,
     ToCss,
     ToShmem,
-    ToResolvedValue,
     Parse,
 )]
 #[repr(C, u8)]