servo/servo — pull request #3990 by pcwalton (2014-11-15 01:40:05+00:00)
Title: gfx: Rewrite display list construction to make stacking-contexts more first-class.

This implements the scheme described here:

https://groups.google.com/forum/#!topic/mozilla.dev.servo/sZVPSfPVfkg

This commit changes Servo to generate one display list per stacking
context instead of one display list per layer. This is purely a
refactoring; there are no functional changes. Performance is essentially
the same as before. However, there should be numerous future benefits
that this is intended to allow for:

* It makes the code simpler to understand because the "new layer needed"
vs. "no new layer needed" code paths are more consolidated.

* It makes it easy to support CSS properties that did not fit into our
previous flat display list model (without unconditionally layerizing
them):

o `opacity` should be easy to support because the stacking context
provides the higher-level grouping of display items to which opacity
is to be applied.

o `transform` can be easily supported because the stacking context
provides a place to stash the transformation matrix. This has the side
benefit of nicely separating the transformation matrix from the
clipping regions.

* The `flatten` logic is now O(1) instead of O(n) and now only needs to
be invoked for pseudo-stacking contexts (right now: just floats),
instead of for every stacking context.

* Layers are now a proper tree instead of a flat list as far as layout
is concerned, bringing us closer to a production-quality
compositing/layers framework.

* This commit opens the door to incremental display list construction at
the level of stacking contexts.

Future performance improvements could come from optimizing allocation of
display list items, and, of course, incremental display list
construction.

r? @glennw
f? @mrobinson @cgaebel