This improves Servo's performance on large pages.
Please double-check the logic when it comes to nested layers—I'm sure I've messed up some of the geometry calculations :)
r? @glennw
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The [current implementation](http://mxr.mozilla.org/servo/source/components/script/dom/element.rs#666) is wrong as it always tries to cast the furthest ancestor into an element.
When the method is invoked on an element that is in the document tree, this will be the `Document` node, which is not an element. The ```last().map(ElementCast::to_temporary)``` returns Some(None) hence the method will return ```None```, while the correct behaviour is to return the ```html``` node.
This PR interprets the line ```"furthest ancestor element node of whatever node is being discussed"``` in the spec to mean ```"find the furthest ancestor that is an element node"```.
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This property determines the background positioning area, that is the position of
the origin of an image specified using the 'background-image' CSS property.
'background-origin' is ignored when background-attachment is fixed.
Spec: http://dev.w3.org/csswg/css-backgrounds-3/#background-originFixes#6045.
This relies on a global webdriverCallback function, which is visible to content.
Obviously that's not a long term solution for a number of reasons, but it allows
us to experiment for now
The basic idea is it's safe to output an image for reftest by testing:
- That the compositor doesn't have any animations active.
- That the compositor is not waiting on any outstanding paint messages to arrive.
- That the script tasks are "idle" and therefore won't cause reflow.
- This currently means page loaded, onload fired, reftest-wait not active, first reflow triggered.
- It could easily be expanded to handle pending timers etc.
- That the "epoch" that the layout tasks have last laid out after script went idle, is reflected by the compositor in all visible layers for that pipeline.
This implements a simple load-tracking system and tracks stylesheet loads as an example of how it fits together. This is a simplified and rebased version of #3714; I do not believe that the main thrust of hsivonen's comments (related to tracking navigation in browsing contexts) affect this part of the work.
r? @Ms2ger
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