Subpixel rendering: roundToDevicePixel() snaps to wrong value.

[zalan]

RenderLayer::paintForegroundForFragmentsWithPhase() gets a fragment with a foregroundRect of, say:

(WebCore::LayoutRect) $23 = (m_location = { x = -96452px (-6172928), y = 0px (0) }, m_size = { width = 146px (9344), height = 310px (19840) })

where -96452 is the left edge of the layer (negative, because we're in vertical-rl content).

snapRectToDevicePixels() is converting that to:

(const WebCore::FloatRect) $24 = {
  m_location = (m_x = -96451.5, m_y = 0)
  m_size = (m_width = 146, m_height = 310)
}

[zalan]

rdar://problem/18509840

[zalan]

Created attachment 255471 [details]
Patch

[zalan]

Created attachment 255487 [details]
Patch

[WebKit Commit Bot]

Comment on attachment 255487 [details]
Patch

Clearing flags on attachment: 255487

Committed r185916: <http://trac.webkit.org/changeset/185916>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.

[Darin Adler]

Comment on attachment 255471 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=255471&action=review

> Source/WebCore/platform/LayoutUnit.h:875
>  inline float roundToDevicePixel(LayoutUnit value, const float pixelSnappingFactor, bool needsDirectionalRounding = false)

The const here adds nothing and should be removed.

[zalan]

(In reply to comment #6)
> Comment on attachment 255471 [details]
> Patch
> 
> View in context:
> https://bugs.webkit.org/attachment.cgi?id=255471&action=review
> 
> > Source/WebCore/platform/LayoutUnit.h:875
> >  inline float roundToDevicePixel(LayoutUnit value, const float pixelSnappingFactor, bool needsDirectionalRounding = false)
> 
> The const here adds nothing and should be removed.
Indeed. Thanks.

[zalan]

Addressing post-review comment: 
http://trac.webkit.org/changeset/186128