The test case has a fixed item A inside a scrollframe B which is inside
a reference frame C which is inside the root scrollframe D. The
ClipManager code currently uses D's scrollid as the scrolling ancestor
for A, because the gecko display list's ASR is set up that way. However,
we really want to set C as the scrolling ancestor, because otherwise the
item A gets hoisted out of C and the transform from C doesn't get
applied to it. This patch ensures that when we enter C, we install an
override so that anything that would have used D's scrollid ends up
using C's, which results in the correct behaviour.
MozReview-Commit-ID: 31tscfT4xWW
--HG--
extra : rebase_source : 03df2fa5519b2592a2c9f598af0f3f1500773718
The clip chain API in webrender allows us to build the clip state in WR
so that it matches the gecko display list more closely. This patch throws
away ScrollingLayersHelper.* and introduces ClipManager.* which pushes
the clip state to WR using the new method. A quick summary of the new
method is below.
Each display item in gecko has a DisplayItemClipChain which is a chain
of individual clips. The individual clips are defined in WR, and the
clip ids for those clips are put into a WR clip chain using the new
define_clip_chain API. Furthermore, each clip chain can also have a
parent chain, which is used to link a DisplayItemClipChain to the parent
display item's DisplayItemClipChain. This allows the WR clip state to
closely match the structure of the gecko display list clip state,
resulting in more correct behaviour.
There are a few other major changes that are lumped into this patch and
that were tricky to separate into their own patches:
- The collapsing of WrScrollId and WrStickyId into WrClipId. On the WR
side all the clip ids are treated the same anyway. Trying to preserve
the arbitrary distinction on the gecko side was resulting in
increasingly convoluted code, with different kinds of Variant<..>
types in the method signatures. It was much simpler and resulted in a
bunch of code deletion to just collapse the types.
- Moving the "override" mechanism from WebRenderAPI to ClipManager. The
override mechanism (explained in ClipManager.h) was simplified by
moving it into ClipManager, because it removed the need for tracking
additional clip stack state in WebRenderAPI.
MozReview-Commit-ID: GGbdFyJGprK
--HG--
extra : rebase_source : baa56ff179e917b0ab5a5c186a3a415761f8050a
Kartikaya Gupta