Bug 23104: Add a default line height compensation

Many fonts have issues with their vertical metrics. they are used to influence the height of ascenders and depth of descenders. Gecko uses it to calculate the line height (font height + ascender + descender), however because of that idiosyncratic behavior across multiple operating systems, it can be used to identify the user's OS. The solution proposed in the patch uses a default factor to be multiplied with the font size, simulating the concept of ascender and descender. This way all operating systems will have the same line height only and only if the frame is outside the chrome.

Bug 23104: Add a default line height compensation
0ac9d766 · Igor Oliveira · Pier Angelo Vendrame · 1665d110 · 0ac9d766 · 0ac9d766
Verified Commit 0ac9d766 authored Dec 10, 2017 by Igor Oliveira Committed by Pier Angelo Vendrame May 29, 2023
--- a/layout/generic/ReflowInput.cpp
+++ b/layout/generic/ReflowInput.cpp
@@ -34,6 +34,7 @@
 #include "nsTableCellFrame.h"
 #include "nsTableFrame.h"
 #include "StickyScrollContainer.h"
+#include "nsContentUtils.h"

 using namespace mozilla;
 using namespace mozilla::css;
@@ -2715,7 +2716,8 @@ void ReflowInput::CalculateBlockSideMargins() {

 // For risk management, we use preference to control the behavior, and
 // eNoExternalLeading is the old behavior.
-static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics) {
+static nscoord GetNormalLineHeight(nsIContent* aContent,
+                                   nsFontMetrics* aFontMetrics) {
  MOZ_ASSERT(nullptr != aFontMetrics, "no font metrics");

  nscoord normalLineHeight;
@@ -2723,6 +2725,12 @@ static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics) {
  nscoord externalLeading = aFontMetrics->ExternalLeading();
  nscoord internalLeading = aFontMetrics->InternalLeading();
  nscoord emHeight = aFontMetrics->EmHeight();
+
+  if (nsContentUtils::ShouldResistFingerprinting() &&
+      !aContent->IsInChromeDocument()) {
+    return NSToCoordRound(emHeight * NORMAL_LINE_HEIGHT_FACTOR);
+  }
+
  switch (GetNormalLineHeightCalcControl()) {
    case eIncludeExternalLeading:
      normalLineHeight = emHeight + internalLeading + externalLeading;
@@ -2740,7 +2748,8 @@ static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics) {
  return normalLineHeight;
 }

-static inline nscoord ComputeLineHeight(const ComputedStyle* aComputedStyle,
+static inline nscoord ComputeLineHeight(nsIContent* aContent,
+                                        const ComputedStyle* aComputedStyle,
                                        nsPresContext* aPresContext,
                                        nscoord aBlockBSize,
                                        float aFontSizeInflation) {
@@ -2769,7 +2778,7 @@ static inline nscoord ComputeLineHeight(const ComputedStyle* aComputedStyle,

  RefPtr<nsFontMetrics> fm = nsLayoutUtils::GetFontMetricsForComputedStyle(
      aComputedStyle, aPresContext, aFontSizeInflation);
-  return GetNormalLineHeight(fm);
+  return GetNormalLineHeight(aContent, fm);
 }

 nscoord ReflowInput::GetLineHeight() const {
@@ -2806,7 +2815,7 @@ nscoord ReflowInput::CalcLineHeight(nsIContent* aContent,
                                    float aFontSizeInflation) {
  MOZ_ASSERT(aComputedStyle, "Must have a ComputedStyle");

-  nscoord lineHeight = ComputeLineHeight(aComputedStyle, aPresContext,
+  nscoord lineHeight = ComputeLineHeight(aContent, aComputedStyle, aPresContext,
                                         aBlockBSize, aFontSizeInflation);

  NS_ASSERTION(lineHeight >= 0, "ComputeLineHeight screwed up");
@@ -2819,7 +2828,7 @@ nscoord ReflowInput::CalcLineHeight(nsIContent* aContent,
    if (!lh.IsNormal()) {
      RefPtr<nsFontMetrics> fm = nsLayoutUtils::GetFontMetricsForComputedStyle(
          aComputedStyle, aPresContext, aFontSizeInflation);
-      nscoord normal = GetNormalLineHeight(fm);
+      nscoord normal = GetNormalLineHeight(aContent, fm);
      if (lineHeight < normal) {
        lineHeight = normal;
      }

--- a/layout/generic/test/mochitest.ini
+++ b/layout/generic/test/mochitest.ini
@@ -150,3 +150,4 @@ support-files =
    file_reframe_for_lazy_load_image.html
 [test_bug1655135.html]
 [test_bug1756831.html]
+[test_tor_bug23104.html]
--- a/layout/generic/test/test_tor_bug23104.html
+++ b/layout/generic/test/test_tor_bug23104.html
+<!DOCTYPE HTML>
+<meta charset="UTF-8">
+<html>
+<head>
+  <title>Test for Tor Bug #23104: CSS line-height reveals the platform Tor browser is running</title>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="application/javascript" src="/tests/SimpleTest/SpawnTask.js"></script>
+  <style type="text/css">
+    span {
+      background-color: #000;
+      color: #fff;
+      font-size: 16.5px;
+    }
+  </style>
+</head>
+<body>
+<span id="test1">Test1</span>
+<span id="test2">كلمة</span>
+<span id="test3">ação</span>
+<script>
+
+let setPref = async function (key, value) {
+  await SpecialPowers.pushPrefEnv({"set": [[key, value]]});
+}
+
+function getStyle(el, styleprop) {
+  el = document.getElementById(el);
+  return document.defaultView.getComputedStyle(el, null).getPropertyValue(styleprop);
+}
+
+function validateElement(elementName, isFingerprintResistent) {
+  var fontSize = getStyle(elementName, 'font-size');
+  var lineHeight = getStyle(elementName, 'line-height');
+  var validationCb = isFingerprintResistent ? is : isnot;
+  validationCb(parseFloat(lineHeight), Math.round(parseFloat(fontSize)) * 1.2, 'Line Height validation');
+}
+
+add_task(async function() {
+  await setPref("layout.css.line-height.normal-as-resolved-value.enabled", false);
+  for (let resistFingerprintingValue of [true, false]) {
+    await setPref("privacy.resistFingerprinting", resistFingerprintingValue);
+    for (let elementId of ['test1', 'test2', 'test3']) {
+      validateElement(elementId, resistFingerprintingValue);
+    }
+  }
+});
+
+</script>
+</body>
+</html>