nsLayoutUtils::SetDisplayPortMargins (and nsIDOMWindowUtils::SetDisplayPortMarginsForElement which wraps it) take alignment arguments (one for each axis) and store them along with the margins; nsLayoutUtils::GetDisplayPort then aligns the displayport assembled from the base rect and the margins to these values. The purpose of the alignment is to allow aligning the displayport to tile boundaries when tiling is enabled. When tiling is disabled, we pass alignment values of 1. This is undesirable since nsLayoutUtils::GetDisplayPort expands the displayport by 1 pixel prior to aligning it to push it to the next tile boundary. We don't want this expansion when tiling is disabled. Instead, we should use alignment values of 0 when tiling is disabled. nsLayoutUtils::GetDisplayPort already skips the alignment code if the alignment is 0, which is what we want. [1] http://dxr.mozilla.org/mozilla-central/source/widget/xpwidgets/APZCCallbackHelper.cpp?from=APZCCallbackHelper.cpp#233 [2] http://dxr.mozilla.org/mozilla-central/source/widget/xpwidgets/APZCCallbackHelper.cpp?from=APZCCallbackHelper.cpp#288

Applies on top of the patches from bug 982141, which add another call site to SetDisplayPortMargins.