Snap inline box boundaries to layout units during line breaking.

Snap inline box boundaries to layout units during line breaking.

When the line box tree is created, InlineBox boundaries are snapped
to LayoutUnit's, and the paint code uses those boundaries to
calculate offsets.  Those snapped coordinates may be slightly
different from the un-snapped coordinates used in the line breaking
code.

This doesn't usually cause noticeable problems, but once place it
can show up is in calculating tab widths.  The size of a tab
character is "the distance between the current offset and the next
tab stop."  If the value of "the current offset" changes between
line breaking and painting, and the difference happens to straddle a
tab position, then layout and painting will use different values for
the size of the tab character.

BUG=526127
Committed: https://crrev.com/27f50dfe5c9b263b25aec4f7b13fb0f90eba3bf7
Cr-Commit-Position: refs/heads/master@{#351368}

[szager1, 2015-09-29 04:04:55]

szager@chromium.org changed reviewers:
+ eae@chromium.org, leviw@chromium.org

[eae, 2015-09-29 14:50:00]

https://codereview.chromium.org/1375803002/diff/1/third_party/WebKit/Source/c...
File third_party/WebKit/Source/core/layout/line/LineWidth.h (right):

https://codereview.chromium.org/1375803002/diff/1/third_party/WebKit/Source/c...
third_party/WebKit/Source/core/layout/line/LineWidth.h:71: void
snapUncommittedWidth() { m_uncommittedWidth =
LayoutUnit(m_uncommittedWidth).toFloat(); }
Is the currentWidth already snapped? What about snapping currentWidth?

[szager1, 2015-09-29 16:48:32]

https://codereview.chromium.org/1375803002/diff/1/third_party/WebKit/Source/c...
File third_party/WebKit/Source/core/layout/line/LineWidth.h (right):

https://codereview.chromium.org/1375803002/diff/1/third_party/WebKit/Source/c...
third_party/WebKit/Source/core/layout/line/LineWidth.h:71: void
snapUncommittedWidth() { m_uncommittedWidth =
LayoutUnit(m_uncommittedWidth).toFloat(); }
On 2015/09/29 14:50:00, eae wrote:
> Is the currentWidth already snapped? What about snapping currentWidth?

Based on conversation with eae: currentWidth is used while laying out words in a
single LayoutText, and at that time, it's expected that offsets are not snapped.
 Snapping currentWidth() isn't obviously wrong, but it's not obviously right
either.  Since the layout tests are passing without that change, I'm going to
leave it out of this patch.

I added a comment to the code about this.

[eae, 2015-09-29 17:15:33]

LGTM

[szager1, 2015-09-29 17:18:23]

The CQ bit was checked by szager@chromium.org

[commit-bot: I haz the power, 2015-09-29 17:19:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1375803002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1375803002/20001

[commit-bot: I haz the power, 2015-09-29 19:05:49]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-29 19:07:25]

Patchset 2 (id:??) landed as
https://crrev.com/27f50dfe5c9b263b25aec4f7b13fb0f90eba3bf7
Cr-Commit-Position: refs/heads/master@{#351368}