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Does WebKit have a clipping bug?

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&#8212;excellent for drawing the
    veil from men&#8217;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&#8212;it was on the twentieth of March, 1888&#8212;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>
like image 352
Quentin Avatar asked Dec 01 '10 14:12

Quentin


2 Answers

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 helvetica as the typeface for sans-serif. But Firefox moves that particular typeface upwards in the line-height box, so the bottom doesn't "stick out". Compare with arial - microsoft's bastardization of helvetica where both browsers let it stick out.

Safari vs Firefox descender sans-serif/arial

I think the solution to your problem is to find a "reasonable" negative margin to offset the content upwards. It seems both helvetica and arial have 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 specific font-size/line-height in your example.

Here's my test code. It shows that the "sticking out" can be much worse for geneva and verdana.

<!DOCTYPE HTML> 
<html lang="en"> 
    <head> 
        <meta charset="utf-8"/> 
        <title>descender issue</title> 
        <style> 

          * {
            margin: 0;
            padding: 0;
          }

          .content {
            margin-bottom: 30px;
            background: #ffdd88;
            font-size: 190px;
            line-height: 210px;
          }

          #content1 {
            font-family: sans-serif;
          }

          #content2 {
            font-family: arial;
          }

          #content3 {
            font-family: geneva;
          }


          #content4 {
            font-family: helvetica;
          }

          #content5 {
            font-family: 'trebuchet ms';
          }

          #content6 {
            font-family: verdana;
          }

          #content7 {
            font-family: serif;
          }

          #content8 {
            font-family: times;
          }

        </style> 
    </head> 
    <body> 
      <h1>descender issue</h1> 
      sans-serif
        <div class="content" id="content1"> 
          lfgjpq
        </div> 
      arial
        <div class="content" id="content2"> 
          lfgjpq
        </div> 
      geneva
        <div class="content" id="content3"> 
          lfgjpq
        </div> 
      helvetica
        <div class="content" id="content4"> 
          lfgjpq
        </div> 
      trebuchet ms
        <div class="content" id="content5"> 
          lfgjpq
        </div> 
      verdana
        <div class="content" id="content6"> 
          lfgjpq
        </div> 
      serif
        <div class="content" id="content7"> 
          lfgjpq
        </div> 
      times
        <div class="content" id="content8"> 
          lfgjpq
        </div> 
    </body> 
</html>
like image 121
Martin Algesten Avatar answered Nov 07 '22 15:11

Martin Algesten


Have you tried increasing line height by 1px? I tried it in Chrome and it works:

line-height: 22px;
height: 220px; /* A multiple of line height */

and:

document.getElementById('wrapper').scrollTop += 220;

Alternatively you can reduce the font size by 1px which also works for me:

/* alternative solution: */
font-size: 18px;
like image 38
slebetman Avatar answered Nov 07 '22 15:11

slebetman