From 286130b051430045d43d8a5917cb36f2f8f9b8cf Mon Sep 17 00:00:00 2001
From: Glenn Watson <gw@intuitionlibrary.com>
Date: Wed, 17 Feb 2016 09:42:21 +1000
Subject: [PATCH] Ensure when calculating font metrics that the total line
 height matches requested line height.

This fixes rounding accuracy issues that could result in layout producing results off by a small number of Au.
---
 components/layout/inline.rs                              | 9 +++++++--
 .../html/transform-input-017.htm.ini                     | 5 -----
 .../html/transform-input-018.htm.ini                     | 4 +++-
 .../css21_dev/html4/position-relative-035.htm.ini        | 4 +++-
 4 files changed, 13 insertions(+), 9 deletions(-)
 delete mode 100644 tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini

diff --git a/components/layout/inline.rs b/components/layout/inline.rs
index d20fb2224a1..1516fd636eb 100644
--- a/components/layout/inline.rs
+++ b/components/layout/inline.rs
@@ -1928,9 +1928,14 @@ impl InlineMetrics {
     #[inline]
     pub fn from_font_metrics(font_metrics: &FontMetrics, line_height: Au) -> InlineMetrics {
         let leading = line_height - (font_metrics.ascent + font_metrics.descent);
+        // Calculating the half leading here and then using leading - half_leading
+        // below ensure that we don't introduce any rounding accuracy issues here.
+        // The invariant is that the resulting total line height must exactly
+        // equal the requested line_height.
+        let half_leading = leading.scale_by(0.5);
         InlineMetrics {
-            block_size_above_baseline: font_metrics.ascent + leading.scale_by(0.5),
-            depth_below_baseline: font_metrics.descent + leading.scale_by(0.5),
+            block_size_above_baseline: font_metrics.ascent + half_leading,
+            depth_below_baseline: font_metrics.descent + leading - half_leading,
             ascent: font_metrics.ascent,
         }
     }
diff --git a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini b/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini
deleted file mode 100644
index 2fbc2d5d73d..00000000000
--- a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[transform-input-017.htm]
-  type: reftest
-  expected:
-    if os == "linux": PASS
-    FAIL
diff --git a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini b/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini
index 23299ad9e96..1c005ae8a57 100644
--- a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini
+++ b/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini
@@ -1,3 +1,5 @@
 [transform-input-018.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "mac": PASS
+    FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/position-relative-035.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/position-relative-035.htm.ini
index 9cb4b4c28f7..5587e47e1bc 100644
--- a/tests/wpt/metadata-css/css21_dev/html4/position-relative-035.htm.ini
+++ b/tests/wpt/metadata-css/css21_dev/html4/position-relative-035.htm.ini
@@ -1,3 +1,5 @@
 [position-relative-035.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "linux": PASS
+    FAIL