92163 – flexitems can overflow the flexbox due to rounding

RESOLVED FIXED 92163

flexitems can overflow the flexbox due to rounding

https://bugs.webkit.org/show_bug.cgi?id=92163

Summary flexitems can overflow the flexbox due to rounding

Tony Chang

Reported 2012-07-24 14:35:39 PDT

Flexbox items don't seem to be getting subpixel positions when rounding. See the URL for an example. It's a flexbox that is 101 pixels wide and has 2 children, each should be the same size (50.5px each). Instead, they are both 51px. For reference, in RenderFlexibleBox.cpp, we use setOverrideLogicalContent{Width,Height}() to set the width/height of children after we have computed the desired value. We use setLocation() to position each child. This happens in RenderFlexibleBox::layoutAndPlaceChildren.

Attachments
proof of concept patch (1.85 KB, patch) 2012-07-24 15:33 PDT, Emil A Eklund	no flags	Details Formatted Diff Diff
Patch (7.84 KB, patch) 2012-07-25 16:45 PDT, Tony Chang	no flags	Details Formatted Diff Diff
Show Obsolete (1) View All Add attachment proposed patch, testcase, etc.

Emil A Eklund

Comment 1 2012-07-24 15:12:09 PDT

It would be interesting to know where the rounding happens as both values seems to be rounded independently of each other. Looking into it quickly it seems like the values are rounded before being passed into the layoutAndPlaceChildren method in the childSizes vector.

Emil A Eklund

Comment 2 2012-07-24 15:14:37 PDT

The problem might be the following code in RenderFlexibleBox::resolveFlexibleLengths: if (availableFreeSpace > 0 && totalFlexGrow > 0 && flexSign == PositiveFlexibility && isfinite(totalFlexGrow)) childSize += static_cast<int>(lroundf(availableFreeSpace * child->style()->flexGrow() / totalFlexGrow)); else if (availableFreeSpace < 0 && totalWeightedFlexShrink > 0 && flexSign == NegativeFlexibility && isfinite(totalWeightedFlexShrink)) childSize += static_cast<int>(lroundf(availableFreeSpace * child->style()->flexShrink() * preferredChildSize / totalWeightedFlexShrink));

Tony Chang

Comment 3 2012-07-24 15:18:29 PDT

(In reply to comment #2) > The problem might be the following code in RenderFlexibleBox::resolveFlexibleLengths: > > if (availableFreeSpace > 0 && totalFlexGrow > 0 && flexSign == PositiveFlexibility && isfinite(totalFlexGrow)) > childSize += static_cast<int>(lroundf(availableFreeSpace * child->style()->flexGrow() / totalFlexGrow)); > else if (availableFreeSpace < 0 && totalWeightedFlexShrink > 0 && flexSign == NegativeFlexibility && isfinite(totalWeightedFlexShrink)) > childSize += static_cast<int>(lroundf(availableFreeSpace * child->style()->flexShrink() * preferredChildSize / totalWeightedFlexShrink)); That seems likely. I'll see if I can track down the history of the lroundf usage.

Emil A Eklund

Comment 4 2012-07-24 15:20:11 PDT

turns out it is not quite that easy. Getting rid of that rounding makes both boxes return 50.5 and prevents the yellow box from being painted outside the container but it does cause the blue and yellow boxes to overlap by one pixel.

Tony Chang

Comment 5 2012-07-24 15:28:39 PDT

The lroundf is from the initial commit. It doesn't seem necessary now, but we'll have to do something more to prevent the overlapping content.

Emil A Eklund

Comment 6 2012-07-24 15:31:09 PDT

Changing childLocation on line 1022 from IntPoint to LayoutPoint, in addition to getting rid of the rounding, seems to fix the issue. Might be other edge cases though.

Emil A Eklund

Comment 7 2012-07-24 15:33:18 PDT

Created attachment 154156 [details] proof of concept patch

Tony Chang

Comment 8 2012-07-24 15:35:28 PDT

(In reply to comment #6) > Changing childLocation on line 1022 from IntPoint to LayoutPoint, in addition to getting rid of the rounding, seems to fix the issue. Might be other edge cases though. Magical! When I make both those changes, all the flexbox tests pass for me. I'll add a layout test for this and upload as a new patch. Thanks for the help!

Emil A Eklund

Comment 9 2012-07-24 15:37:05 PDT

(In reply to comment #8) > (In reply to comment #6) > > Changing childLocation on line 1022 from IntPoint to LayoutPoint, in addition to getting rid of the rounding, seems to fix the issue. Might be other edge cases though. > > Magical! When I make both those changes, all the flexbox tests pass for me. I'll add a layout test for this and upload as a new patch. Thanks for the help! Nice! Glad I was able to help.

Tony Chang

Comment 10 2012-07-25 16:45:24 PDT

Created attachment 154482 [details] Patch

Levi Weintraub

Comment 11 2012-07-25 18:17:16 PDT

Comment on attachment 154482 [details] Patch LGTM

WebKit Review Bot

Comment 12 2012-07-25 19:25:10 PDT

Comment on attachment 154482 [details] Patch Clearing flags on attachment: 154482 Committed r123696: <http://trac.webkit.org/changeset/123696>

WebKit Review Bot

Comment 13 2012-07-25 19:25:13 PDT

All reviewed patches have been landed. Closing bug.