Bug 125757

Summary:

Subpixel layout: Rounding padding vales pushes the content 1px off.

Product:

WebKit

Reporter:

zalan <zalan>

Component:

Layout and Rendering

Assignee:

zalan <zalan>

Status:

RESOLVED INVALID

Severity:

Normal

CC:

hyatt, jonlee, koivisto, simon.fraser, webkit-bug-importer

Priority:

Keywords:

InRadar

Version:

528+ (Nightly build)

Hardware:

Unspecified

OS:

Unspecified

Bug Depends on:

Bug Blocks:

126283

Attachments:

Description	Flags
test case	none
screenshot(wikipedia)	none
regressed test case (wikipedia)	none
Testing FF's rendering.	none

Description zalan 2013-12-15 12:22:46 PST

<html>
<head>
<style>
  body {
    font-size: 99%;
    padding-left: 0.5em;
  }
</style>
</head>
<body>This line is pushed to left by 1px. (body's padding-left is 0.5em with font-size: 99%.)</body>
</html>

Comment 1 zalan 2013-12-15 12:23:50 PST

Created attachment 219279 [details]
test case

Comment 2 zalan 2013-12-15 12:26:19 PST

<rdar://problem/15665134>

Comment 3 zalan 2013-12-16 14:31:54 PST

This is the result of clamping float values at LayoutUnit(float) to int when minimumValueForLength() is called.
computed minimum value: 6.8;
subpixel on: LayoutUnit value -> 435 (which eventually gets pixelsnapped to 7px)
subpixel off: LayoutUnit value -> 6 as the result of the clamping.
It mainly (most visibly) effects paddings and margins, but it can push pixels off by 1 at other places too (like with text-indent, width properties)

div {
    font-size: 99%;
    text-indent: 1em;
}


div {
    font-size: 99%;
    width: 1em;
    display: inline-block;
}

Comment 4 Simon Fraser (smfr) 2013-12-18 11:26:11 PST

I think it's OK to have a behavior change with subpixel when authors are using percentages. We should check Firefox behavior.

Comment 5 zalan 2013-12-19 17:00:45 PST

Created attachment 219704 [details]
screenshot(wikipedia)

Rounding these values would also regress some content including wikipedia.

<html>
<head>
<style>
  div {
    margin-top: -1.4em;
    margin-bottom: 1.4em 
  }
</style>
</head>
<body>
<div></div>
foo
</body>
</html>

Comment 6 zalan 2013-12-19 17:01:22 PST

Created attachment 219705 [details]
regressed test case (wikipedia)

Comment 7 zalan 2014-01-14 15:06:30 PST

Created attachment 221209 [details]
Testing FF's rendering.

The new off-by-one rendering matches FF's (v26.0) rendering. (top to bottom: subpixel on, FF, subpixel off)