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.

Bug 23104: Add a default line height compensation
badd9487 · Igor Oliveira · Pier Angelo Vendrame · 7193a6ae · badd9487 · badd9487
Verified Commit badd9487 authored Dec 10, 2017 by Igor Oliveira Committed by Pier Angelo Vendrame Jun 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;
@@ -2727,7 +2728,8 @@ static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics) {
  nscoord externalLeading = aFontMetrics->ExternalLeading();
  nscoord internalLeading = aFontMetrics->InternalLeading();
  nscoord emHeight = aFontMetrics->EmHeight();
-  if (!internalLeading && !externalLeading) {
+  if ((!internalLeading && !externalLeading) ||
+      nsContentUtils::ShouldResistFingerprinting()) {
    return NSToCoordRound(emHeight * kNormalLineHeightFactor);
  }
  return emHeight + internalLeading + externalLeading;

--- a/layout/generic/test/mochitest.ini
+++ b/layout/generic/test/mochitest.ini
@@ -165,3 +165,4 @@ support-files =
 support-files = !/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
 [test_bug1803209.html]
 [test_bug1831148.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>