svgfe-backdrop-async-scroll.yaml

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# An SVGFE *backdrop* filter mid-async-scroll. APZ has moved the scroll frame to

# 100 but the display list was painted at 50, so the offset baked into the item

# coordinates (external-scroll-offset) lags the sampled offset by an async delta

# of 50. This is the state a wheel or fling produces between paints, and the one

# a subregion offset derived once from the enclosing reference frame cannot

# handle: it would be wrong by exactly the async delta.

# Sampled total scroll is scroll-offset + external-scroll-offset, so 50 + 50 =

# 100 here. Items are authored in painted coordinates (y=50); WebRender adds the

# baked 50 at push time and the scroll node subtracts the current 100, landing

# the red backdrop at y=0. The subregion must make the same round trip, and is

# authored in the same space as the geometry. If it is not normalized it ends up

# 50px low and red shows through the bottom half. See bug 1975275.

root:

  items:

    - type: scroll-frame

      bounds: [0, 0, 100, 100]

      content-size: [100, 1000]

      external-scroll-offset: [0, 50]

      scroll-offset: [0, 50]

      items:

        - type: rect

          color: red

          bounds: [0, 50, 100, 100]

        - type: backdrop-filter

          bounds: [0, 50, 100, 100]

          filters:

            - svgfe: SourceGraphic

            - svgfe: flood

              linear: false

              color: green

              subregion: [0, 50, 100, 50]

            - svgfe: compositeover

              linear: false

              subregion: [0, 50, 100, 100]

              in: 1

              in2: 0