[Webkit-unassigned] [Bug 225961] New: Incorrect rendering of VP9 with transparency

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=225961

            Bug ID: 225961
           Summary: Incorrect rendering of VP9 with transparency
           Product: WebKit
           Version: WebKit Local Build
          Hardware: Unspecified
                OS: Linux
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WPE WebKit
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: nazar at mokrynskyi.com
                CC: bugs-noreply at webkitgtk.org

Upstream GStreamer supports VP8/VP9 with transparency, but some videos are not rendered correctly in WebKit WPE with GStreamer backend, while working fine in Firefox and Chromium.

Example of such video with incorrect rendering is: https://cdn.streamelements.com/uploads/187c102b-ac08-4843-8ac9-4e6be93688b9.webm

Reproduction with GStreamer's wpesrc (this is what I have and use for testing):
LIBGL_ALWAYS_SOFTWARE=true gst-launch-1.0 \
  wpesrc do-timestamp=true location=https://nextcloud.mokrynskyi.com/s/qcoTsNgeLqmAXtg ! \
  video/x-raw,format=BGRA,width=1280,height=720 ! \
  queue ! videoconvert ! autovideosink

Many other videos are rendered properly, so there is something special about that particular file.
NOTE: GStreamer pipeline that takes that video and does some compositing with it also works fine, so it is WPE-specific issue.

Initially reported against gst-plugins-bad: https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/issues/1594

Tested with:
libwpe 1.9.1
wpebackend-fdo 1.9.1
wpewebkit 2.31.1
GStreamer and plugins from master branch

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