The paragraph at http://www.w3.org/TR/css3-break/#break-margins states the bottom margins before a forced break get truncated. When an unforced break occurs, both the top/bottom margins are truncated. Use this bug to track the progress for the implementation of this mechanism in regions, columns and pages.
Created attachment 184806 [details] A test case with bottom margin not getting truncated after a forced break.
Created attachment 184999 [details] WIP Patch
Attachment 184999 [details] did not pass style-queue: Failed to run "['Tools/Scripts/check-webkit-style', '--diff-files', u'LayoutTests/fast/block/margin-collapse/webkit-margin-collapse-siblings.html', u'Source/WebCore/rendering/RenderBlock.cpp']" exit_code: 1 Source/WebCore/rendering/RenderBlock.cpp:2030: When wrapping a line, only indent 4 spaces. [whitespace/indent] [3] Total errors found: 1 in 2 files If any of these errors are false positives, please file a bug against check-webkit-style.
(In reply to comment #2) > Created an attachment (id=184999) [details] > Patch Very early version of the patch showing the approach to fix the -webkit-margins-collapse properties. Unfortunately there's little to nothing specified about this property http://developer.apple.com/library/safari/#documentation/AppleApplications/Reference/SafariCSSRef/Articles/StandardCSSProperties.html#//apple_ref/css/property/-webkit-margin-collapse The test is supposed to cover all the possible combinations of that property between siblings. A reference page will pair it in the final patch. I have a feeling all this logic can be somehow simplified. I'll give this idea a spin in the next version.
Comment on attachment 184999 [details] WIP Patch Attachment 184999 [details] did not pass mac-ews (mac): Output: http://queues.webkit.org/results/16152714 New failing tests: compositing/checkerboard.html compositing/geometry/abs-position-inside-opacity.html compositing/geometry/bounds-clipped-composited-child.html compositing/iframes/become-overlapped-iframe.html animations/3d/matrix-transform-type-animation.html compositing/iframes/connect-compositing-iframe-delayed.html compositing/direct-image-compositing.html compositing/iframes/connect-compositing-iframe2.html animations/additive-transform-animations.html compositing/columns/composited-in-paginated.html compositing/layers-inside-overflow-scroll.html compositing/generated-content.html compositing/geometry/composited-in-columns.html compositing/iframes/connect-compositing-iframe3.html accessibility/lists.html compositing/images/direct-image-background-color.html compositing/geometry/ancestor-overflow-change.html compositing/geometry/bounds-ignores-hidden-dynamic-negzindex.html compositing/iframes/connect-compositing-iframe.html compositing/iframes/become-composited-nested-iframes.html compositing/geometry/clipping-foreground.html compositing/clip-child-by-non-stacking-ancestor.html compositing/iframes/composited-parent-iframe.html compositing/compositing-visible-descendant.html compositing/overflow-trumps-transform-style.html compositing/iframes/composited-iframe-alignment.html compositing/backing/no-backing-for-clip.html compositing/backing/no-backing-for-clip-overlap.html compositing/backing/no-backing-for-perspective.html compositing/layer-creation/animation-overlap-with-children.html
Comment on attachment 184999 [details] WIP Patch Attachment 184999 [details] did not pass chromium-ews (chromium-xvfb): Output: http://queues.webkit.org/results/16163696 New failing tests: compositing/geometry/abs-position-inside-opacity.html compositing/geometry/bounds-clipped-composited-child.html animations/3d/matrix-transform-type-animation.html compositing/direct-image-compositing.html compositing/backing/no-backing-for-clip-overlap.html animations/additive-transform-animations.html accessibility/svg-remote-element.html compositing/layers-inside-overflow-scroll.html compositing/sibling-positioning.html compositing/backing/no-backing-for-perspective.html compositing/generated-content.html compositing/self-painting-layers.html animations/cross-fade-webkit-mask-box-image.html compositing/flat-with-transformed-child.html compositing/geometry/ancestor-overflow-change.html animations/cross-fade-list-style-image.html compositing/geometry/bounds-ignores-hidden-dynamic-negzindex.html animations/cross-fade-background-image.html compositing/columns/composited-in-paginated.html compositing/geometry/clipping-foreground.html animations/cross-fade-border-image-source.html compositing/clip-child-by-non-stacking-ancestor.html compositing/backface-visibility/backface-visibility-image.html compositing/compositing-visible-descendant.html animations/cross-fade-webkit-mask-image.html compositing/overflow-trumps-transform-style.html compositing/backing/no-backing-for-clip.html compositing/backface-visibility/backface-visibility-simple.html accessibility/svg-bounds.html compositing/backface-visibility/backface-visibility-webgl.html
Comment on attachment 184999 [details] WIP Patch Attachment 184999 [details] did not pass mac-wk2-ews (mac-wk2): Output: http://queues.webkit.org/results/16183410 New failing tests: compositing/checkerboard.html compositing/geometry/abs-position-inside-opacity.html compositing/geometry/bounds-clipped-composited-child.html compositing/iframes/become-overlapped-iframe.html animations/3d/matrix-transform-type-animation.html compositing/iframes/connect-compositing-iframe-delayed.html compositing/direct-image-compositing.html compositing/iframes/connect-compositing-iframe2.html animations/additive-transform-animations.html accessibility/svg-remote-element.html compositing/layers-inside-overflow-scroll.html compositing/generated-content.html compositing/self-painting-layers.html compositing/iframes/connect-compositing-iframe3.html compositing/backing/no-backing-for-clip-overlap.html accessibility/lists.html compositing/tiled-layers-hidpi.html compositing/geometry/ancestor-overflow-change.html compositing/geometry/bounds-ignores-hidden-dynamic-negzindex.html compositing/columns/composited-in-paginated.html compositing/iframes/become-composited-nested-iframes.html compositing/geometry/clipping-foreground.html compositing/clip-child-by-non-stacking-ancestor.html compositing/iframes/composited-parent-iframe.html compositing/compositing-visible-descendant.html compositing/overflow-trumps-transform-style.html compositing/iframes/composited-iframe-alignment.html compositing/backing/no-backing-for-clip.html accessibility/svg-bounds.html compositing/backing/no-backing-for-perspective.html
I've removed -webkit-margin-collapse in: https://github.com/WebKit/WebKit/commit/42888f9d55e6863c9e1a8110cc164f81a63c1158 Hopefully this should be easier.