NB: I tidied up this question in 2021 to remove a dead link and make the code a live demo. With a cursory check, the problem appears to no longer exist in the current version of Chrome.
Given a region where the line-height and any margins are n, and the region has a height that is a multiple of n, and the scrollTop is increased by multiples of n I find that I get the result I expect in Firefox, Opera and NetFront but in Chrome (Windows), Safari (Mac) and the latest WebKit nightly (Mac) there is some leakage and I see partial lines.
In my actual project (which I can’t share) the effect is quite pronounced, but even in a reduced test case, the bottom of the previous line can be seen peeking out at the top of the box.
Is it possible to avoid this effect? Is this a bug in the WebKit rendering engine that should be reported?
The reduced test case can be seen below. Click on the document a few times to scroll it and not the dots at the top of the box (which are the bottom of the letters of the previous line).
<!DOCTYPE HTML>
<html lang="en">
<head>
<meta charset="utf-8"/>
<title>scrollTop issue</title>
<style>
body {
background-color: white;
color: black;
}
#wrapper {
width: 300px;
font-size: 19px;
font-family: sans-serif;
line-height: 21px;
height: 210px; /* A multiple of line height */
overflow: hidden;
}
#wrapper * {
margin: 0;
padding: 0;
}
#wrapper p {
margin-bottom: 21px; /* Same as line height */
}
</style>
<script>
window.addEventListener('click', function () {
document.getElementById('wrapper').scrollTop += 210;
});
</script>
</head>
<body>
<h1>scrollTop issue</h1>
<div id="wrapper">
<div id="content">
<p>To Sherlock Holmes she is always <i>the</i> woman. I have seldom heard
him mention her under any other name. In his eyes she eclipses
and predominates the whole of her sex. It was not that he felt
any emotion akin to love for Irene Adler. All emotions, and that
one particularly, were abhorrent to his cold, precise but
admirably balanced mind. He was, I take it, the most perfect
reasoning and observing machine that the world has seen, but as a
lover he would have placed himself in a false position. He never
spoke of the softer passions, save with a gibe and a sneer. They
were admirable things for the observer—excellent for drawing the
veil from men’s motives and actions. But for the trained reasoner
to admit such intrusions into his own delicate and finely
adjusted temperament was to introduce a distracting factor which
might throw a doubt upon all his mental results. Grit in a
sensitive instrument, or a crack in one of his own high-power
lenses, would not be more disturbing than a strong emotion in a
nature such as his. And yet there was but one woman to him, and
that woman was the late Irene Adler, of dubious and questionable
memory.
<p>
I had seen little of Holmes lately. My marriage had drifted us
away from each other. My own complete happiness, and the
home-centred interests which rise up around the man who first
finds himself master of his own establishment, were sufficient to
absorb all my attention, while Holmes, who loathed every form of
society with his whole Bohemian soul, remained in our lodgings in
Baker Street, buried among his old books, and alternating from
week to week between cocaine and ambition, the drowsiness of the
drug, and the fierce energy of his own keen nature. He was still,
as ever, deeply attracted by the study of crime, and occupied his
immense faculties and extraordinary powers of observation in
following out those clues, and clearing up those mysteries which
had been abandoned as hopeless by the official police. From time
to time I heard some vague account of his doings: of his summons
to Odessa in the case of the Trepoff murder, of his clearing up
of the singular tragedy of the Atkinson brothers at Trincomalee,
and finally of the mission which he had accomplished so
delicately and successfully for the reigning family of Holland.
Beyond these signs of his activity, however, which I merely
shared with all the readers of the daily press, I knew little of
my former friend and companion.
<p>
One night—it was on the twentieth of March, 1888—I was
returning from a journey to a patient (for I had now returned to
civil practice), when my way led me through Baker Street. As I
passed the well-remembered door, which must always be associated
in my mind with my wooing, and with the dark incidents of the
Study in Scarlet, I was seized with a keen desire to see Holmes
again, and to know how he was employing his extraordinary powers.
His rooms were brilliantly lit, and, even as I looked up, I saw
his tall, spare figure pass twice in a dark silhouette against
the blind. He was pacing the room swiftly, eagerly, with his head
sunk upon his chest and his hands clasped behind him. To me, who
knew his every mood and habit, his attitude and manner told their
own story. He was at work again. He had risen out of his
drug-created dreams and was hot upon the scent of some new
problem. I rang the bell and was shown up to the chamber which
had formerly been in part my own.
</div>
</div>
</body>
</html>
It appears this has to do with descenders (qjpg) “sticking out” down below the line-height box. Firefox and Safari seem to agree on how to do this – characters are allowed to stick out. However by exaggerating the sizes x 10, we notice something interesting for
sans-serif.In Mac OS X, both Safari and Firefox chose
helveticaas the typeface forsans-serif. But Firefox moves that particular typeface upwards in the line-height box, so the bottom doesn’t “stick out”. Compare witharial– microsoft’s bastardization ofhelveticawhere both browsers let it stick out.I think the solution to your problem is to find a “reasonable” negative margin to offset the content upwards. It seems both
helveticaandarialhave some “wiggle room” at the top of the box. I would use#wrapper #content { margin-top: -1px; }(selector extra strong to overcome#wrapper *) for the specificfont-size/line-heightin your example.Here’s my test code. It shows that the “sticking out” can be much worse for
genevaandverdana.