From f19df142f8dce515dfba244d8bc9eaf40683da2e Mon Sep 17 00:00:00 2001
From: Oriol Brufau <obrufau@igalia.com>
Date: Fri, 5 May 2023 23:31:01 +0200
Subject: [PATCH] Don't go through app-units-ratio to convert absolute lengths
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This is a backport of https://phabricator.services.mozilla.com/D143942,
by Emilio Cobos Álvarez.

This doesn't change behavior on its own, but the current code introduces
some minor floating point error which we can avoid, and which would
cause failures with the patch for #29696.
---
 components/style/values/specified/font.rs   |  4 +--
 components/style/values/specified/length.rs | 38 ++++++++++-----------
 2 files changed, 20 insertions(+), 22 deletions(-)

diff --git a/components/style/values/specified/font.rs b/components/style/values/specified/font.rs
index 5f45d81d83d..bda757440bb 100644
--- a/components/style/values/specified/font.rs
+++ b/components/style/values/specified/font.rs
@@ -15,7 +15,7 @@ use crate::values::computed::{CSSPixelLength, Context, ToComputedValue};
 use crate::values::generics::font::VariationValue;
 use crate::values::generics::font::{self as generics, FeatureTagValue, FontSettings, FontTag};
 use crate::values::generics::NonNegative;
-use crate::values::specified::length::{FontBaseSize, AU_PER_PT, AU_PER_PX};
+use crate::values::specified::length::{FontBaseSize, PX_PER_PT};
 use crate::values::specified::{AllowQuirks, Angle, Integer, LengthPercentage};
 use crate::values::specified::{NoCalcLength, NonNegativeNumber, Number, Percentage};
 use crate::values::CustomIdent;
@@ -2287,7 +2287,7 @@ impl MozScriptMinSize {
     #[inline]
     /// Calculate initial value of -moz-script-min-size.
     pub fn get_initial_value() -> Length {
-        Length::new(DEFAULT_SCRIPT_MIN_SIZE_PT as f32 * (AU_PER_PT / AU_PER_PX))
+        Length::new(DEFAULT_SCRIPT_MIN_SIZE_PT as f32 * PX_PER_PT)
     }
 }
 
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index f706b401b9d..7c03e8dce42 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -32,20 +32,18 @@ pub use super::image::Image;
 pub use super::image::{EndingShape as GradientEndingShape, Gradient};
 pub use crate::values::specified::calc::CalcLengthPercentage;
 
-/// Number of app units per pixel
-pub const AU_PER_PX: CSSFloat = 60.;
-/// Number of app units per inch
-pub const AU_PER_IN: CSSFloat = AU_PER_PX * 96.;
-/// Number of app units per centimeter
-pub const AU_PER_CM: CSSFloat = AU_PER_IN / 2.54;
-/// Number of app units per millimeter
-pub const AU_PER_MM: CSSFloat = AU_PER_IN / 25.4;
-/// Number of app units per quarter
-pub const AU_PER_Q: CSSFloat = AU_PER_MM / 4.;
-/// Number of app units per point
-pub const AU_PER_PT: CSSFloat = AU_PER_IN / 72.;
-/// Number of app units per pica
-pub const AU_PER_PC: CSSFloat = AU_PER_PT * 12.;
+/// Number of pixels per inch
+pub const PX_PER_IN: CSSFloat = 96.;
+/// Number of pixels per centimeter
+pub const PX_PER_CM: CSSFloat = PX_PER_IN / 2.54;
+/// Number of pixels per millimeter
+pub const PX_PER_MM: CSSFloat = PX_PER_IN / 25.4;
+/// Number of pixels per quarter
+pub const PX_PER_Q: CSSFloat = PX_PER_MM / 4.;
+/// Number of pixels per point
+pub const PX_PER_PT: CSSFloat = PX_PER_IN / 72.;
+/// Number of pixels per pica
+pub const PX_PER_PC: CSSFloat = PX_PER_PT * 12.;
 
 /// A font relative length.
 #[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, ToCss, ToShmem)]
@@ -407,12 +405,12 @@ impl AbsoluteLength {
 
         let pixel = match *self {
             AbsoluteLength::Px(value) => value,
-            AbsoluteLength::In(value) => value * (AU_PER_IN / AU_PER_PX),
-            AbsoluteLength::Cm(value) => value * (AU_PER_CM / AU_PER_PX),
-            AbsoluteLength::Mm(value) => value * (AU_PER_MM / AU_PER_PX),
-            AbsoluteLength::Q(value) => value * (AU_PER_Q / AU_PER_PX),
-            AbsoluteLength::Pt(value) => value * (AU_PER_PT / AU_PER_PX),
-            AbsoluteLength::Pc(value) => value * (AU_PER_PC / AU_PER_PX),
+            AbsoluteLength::In(value) => value * PX_PER_IN,
+            AbsoluteLength::Cm(value) => value * PX_PER_CM,
+            AbsoluteLength::Mm(value) => value * PX_PER_MM,
+            AbsoluteLength::Q(value) => value * PX_PER_Q,
+            AbsoluteLength::Pt(value) => value * PX_PER_PT,
+            AbsoluteLength::Pc(value) => value * PX_PER_PC,
         };
         pixel.min(f32::MAX).max(f32::MIN)
     }