Bug 282126 - Part 4: Use horizontal metrics for ch in vertical mixed/sideways writing modes and for ex always. r=emilio

Depends on D23425

Differential Revision: https://phabricator.services.mozilla.com/D23426

--HG--
extra : moz-landing-system : lando

servo/components/style/font_metrics.rs

@@ -20,6 +20,16 @@ pub struct FontMetrics {
     pub zero_advance_measure: Option<Au>,
 }
 
+/// Type of font metrics to retrieve.
+#[derive(Clone, Debug, PartialEq)]
+pub enum FontMetricsOrientation {
+    /// Get metrics for horizontal or vertical according to the Context's
+    /// writing mode.
+    MatchContext,
+    /// Force getting horizontal metrics.
+    Horizontal,
+}
+
 /// A trait used to represent something capable of providing us font metrics.
 pub trait FontMetricsProvider {
     /// Obtain the metrics for given font family.
@@ -27,7 +37,8 @@ pub trait FontMetricsProvider {
         &self,
         _context: &crate::values::computed::Context,
         _base_size: crate::values::specified::length::FontBaseSize,
-    ) -> FontMetricsQueryResult {
+        _orientation: FontMetricsOrientation,
+    ) -> FontMetrics {
         Default::default()
     }
 

servo/components/style/gecko/wrapper.rs

@@ -20,7 +20,7 @@ use crate::context::{PostAnimationTasks, QuirksMode, SharedStyleContext, UpdateA
 use crate::data::ElementData;
 use crate::dom::{LayoutIterator, NodeInfo, OpaqueNode, TDocument, TElement, TNode, TShadowRoot};
 use crate::element_state::{DocumentState, ElementState};
-use crate::font_metrics::{FontMetrics, FontMetricsProvider};
+use crate::font_metrics::{FontMetrics, FontMetricsOrientation, FontMetricsProvider};
 use crate::gecko::data::GeckoStyleSheet;
 use crate::gecko::selector_parser::{NonTSPseudoClass, PseudoElement, SelectorImpl};
 use crate::gecko::snapshot_helpers;
@@ -1035,6 +1035,7 @@ impl FontMetricsProvider for GeckoFontMetricsProvider {
         &self,
         context: &crate::values::computed::Context,
         base_size: FontBaseSize,
+        orientation: FontMetricsOrientation,
     ) -> FontMetrics {
         let pc = match context.device().pres_context() {
             Some(pc) => pc,
@@ -1056,10 +1057,14 @@ impl FontMetricsProvider for GeckoFontMetricsProvider {
             },
         };
 
+        let vertical_metrics = match orientation {
+            FontMetricsOrientation::MatchContext => wm.is_vertical() && wm.is_upright(),
+            FontMetricsOrientation::Horizontal => false,
+        };
         let gecko_metrics = unsafe {
             bindings::Gecko_GetFontMetrics(
                 pc,
-                wm.is_vertical() && !wm.is_sideways(),
+                vertical_metrics,
                 font.gecko(),
                 size.0,
                 // we don't use the user font set in a media query

servo/components/style/values/specified/length.rs

@@ -7,7 +7,7 @@
 //! [length]: https://drafts.csswg.org/css-values/#lengths
 
 use super::{AllowQuirks, Number, Percentage, ToComputedValue};
-use crate::font_metrics::FontMetrics;
+use crate::font_metrics::{FontMetrics, FontMetricsOrientation};
 use crate::parser::{Parse, ParserContext};
 use crate::properties::computed_value_flags::ComputedValueFlags;
 use crate::values::computed::{self, CSSPixelLength, Context};
@@ -133,8 +133,9 @@ impl FontRelativeLength {
         fn query_font_metrics(
             context: &Context,
             base_size: FontBaseSize,
+            orientation: FontMetricsOrientation,
         ) -> FontMetrics {
-            context.font_metrics_provider.query(context, base_size)
+            context.font_metrics_provider.query(context, base_size, orientation)
         }
 
         let reference_font_size = base_size.resolve(context);
@@ -160,7 +161,12 @@ impl FontRelativeLength {
                     context.rule_cache_conditions.borrow_mut().set_uncacheable();
                 }
                 context.builder.add_flags(ComputedValueFlags::DEPENDS_ON_FONT_METRICS);
-                let metrics = query_font_metrics(context, base_size);
+                // The x-height is an intrinsically horizontal metric.
+                let metrics = query_font_metrics(
+                    context,
+                    base_size,
+                    FontMetricsOrientation::Horizontal
+                );
                 let reference_size = metrics.x_height.unwrap_or_else(|| {
                     // https://drafts.csswg.org/css-values/#ex
                     //
@@ -177,7 +183,18 @@ impl FontRelativeLength {
                     context.rule_cache_conditions.borrow_mut().set_uncacheable();
                 }
                 context.builder.add_flags(ComputedValueFlags::DEPENDS_ON_FONT_METRICS);
-                let metrics = query_font_metrics(context, base_size);
+                // https://drafts.csswg.org/css-values/#ch:
+                //
+                //     Equal to the used advance measure of the “0” (ZERO,
+                //     U+0030) glyph in the font used to render it. (The advance
+                //     measure of a glyph is its advance width or height,
+                //     whichever is in the inline axis of the element.)
+                //
+                let metrics = query_font_metrics(
+                    context,
+                    base_size,
+                    FontMetricsOrientation::MatchContext,
+                );
                 let reference_size = metrics.zero_advance_measure.unwrap_or_else(|| {
                     // https://drafts.csswg.org/css-values/#ch
                     //
@@ -189,7 +206,8 @@ impl FontRelativeLength {
                     //     writing-mode is vertical-rl or vertical-lr and
                     //     text-orientation is upright).
                     //
-                    if context.style().writing_mode.is_vertical() {
+                    let wm = context.style().writing_mode;
+                    if wm.is_vertical() && wm.is_upright() {
                         reference_font_size
                     } else {
                         reference_font_size.scale_by(0.5)