Verified Commit 37c62975 authored by Igor Oliveira's avatar Igor Oliveira Committed by Pier Angelo Vendrame
Browse files

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.
parent cba78274

layout/generic/ReflowInput.cpp

+6 −3
Original line number Diff line number Diff line
@@ -34,6 +34,7 @@ 
#include "nsTableCellFrame.h"

#include "nsTableFrame.h"

#include "StickyScrollContainer.h"

#include "nsContentUtils.h"



using namespace mozilla;

using namespace mozilla::css;
@@ -2723,12 +2724,12 @@ void ReflowInput::CalculateBlockSideMargins() { 
// This is necessary because without this compensation, normal line height might

// look too tight.

constexpr float kNormalLineHeightFactor = 1.2f;

static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics) {

static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics, bool aRFP) {

  MOZ_ASSERT(aFontMetrics, "no font metrics");

  nscoord externalLeading = aFontMetrics->ExternalLeading();

  nscoord internalLeading = aFontMetrics->InternalLeading();

  nscoord emHeight = aFontMetrics->EmHeight();

  if (!internalLeading && !externalLeading) {

  if ((!internalLeading && !externalLeading) || aRFP) {

    return NSToCoordRound(emHeight * kNormalLineHeightFactor);

  }

  return emHeight + internalLeading + externalLeading;
@@ -2768,7 +2769,9 @@ static inline nscoord ComputeLineHeight(const StyleLineHeight& aLh, 
    RefPtr<nsFontMetrics> fm = nsLayoutUtils::GetMetricsFor(

        aPresContext, aIsVertical, &aRelativeToFont, size,

        /* aUseUserFontSet = */ true);

    return GetNormalLineHeight(fm);

    return GetNormalLineHeight(

        fm, aPresContext->Document()->ShouldResistFingerprinting(

                RFPTarget::Unknown));

  }

  // If we don't have a pres context, use a 1.2em fallback.

  size.ScaleBy(kNormalLineHeightFactor);

layout/generic/test/mochitest.ini

+1 −0
Original line number Diff line number Diff line
@@ -164,3 +164,4 @@ support-files = 
[test_scroll_on_display_contents.html]

support-files = !/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js

[test_bug1803209.html]

[test_tor_bug23104.html]

layout/generic/test/test_tor_bug23104.html

0 → 100644
+50 −0
Original line number Diff line number Diff line 
<!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>