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diff -up firefox-99.0/dom/media/mp4/MP4Demuxer.cpp.1670333 firefox-99.0/dom/media/mp4/MP4Demuxer.cpp
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--- firefox-99.0/dom/media/mp4/MP4Demuxer.cpp.1670333	2022-03-31 01:24:44.000000000 +0200
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+++ firefox-99.0/dom/media/mp4/MP4Demuxer.cpp	2022-04-04 09:58:35.606351546 +0200
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@@ -31,6 +31,8 @@ mozilla::LogModule* GetDemuxerLog() { re
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   DDMOZ_LOG(gMediaDemuxerLog, mozilla::LogLevel::Debug, "::%s: " arg, \
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             __func__, ##__VA_ARGS__)
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+extern bool gUseKeyframeFromContainer;
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+
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 namespace mozilla {
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 DDLoggedTypeDeclNameAndBase(MP4TrackDemuxer, MediaTrackDemuxer);
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@@ -394,6 +396,12 @@ already_AddRefed<MediaRawData> MP4TrackD
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           [[fallthrough]];
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         case H264::FrameType::OTHER: {
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           bool keyframe = type == H264::FrameType::I_FRAME;
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+          if (gUseKeyframeFromContainer) {
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+            if (sample->mKeyframe && sample->mKeyframe != keyframe) {
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+              sample->mKeyframe = keyframe;
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+            }
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+            break;
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+          }
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           if (sample->mKeyframe != keyframe) {
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             NS_WARNING(nsPrintfCString("Frame incorrectly marked as %skeyframe "
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                                        "@ pts:%" PRId64 " dur:%" PRId64
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diff -up firefox-99.0/dom/media/platforms/PDMFactory.cpp.1670333 firefox-99.0/dom/media/platforms/PDMFactory.cpp
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--- firefox-99.0/dom/media/platforms/PDMFactory.cpp.1670333	2022-03-31 01:24:44.000000000 +0200
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+++ firefox-99.0/dom/media/platforms/PDMFactory.cpp	2022-04-04 10:09:57.383419125 +0200
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@@ -58,6 +58,8 @@
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 #include <functional>
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+bool gUseKeyframeFromContainer = false;
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+
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 namespace mozilla {
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 #define PDM_INIT_LOG(msg, ...) \
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@@ -495,7 +497,7 @@ void PDMFactory::CreateRddPDMs() {
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 #ifdef MOZ_FFMPEG
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   if (StaticPrefs::media_ffmpeg_enabled() &&
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       StaticPrefs::media_rdd_ffmpeg_enabled() &&
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-      !CreateAndStartupPDM<FFmpegRuntimeLinker>()) {
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+      !(mFFmpegUsed = CreateAndStartupPDM<FFmpegRuntimeLinker>())) {
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     mFailureFlags += GetFailureFlagBasedOnFFmpegStatus(
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         FFmpegRuntimeLinker::LinkStatusCode());
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   }
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@@ -602,8 +604,9 @@ void PDMFactory::CreateDefaultPDMs() {
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   CreateAndStartupPDM<AgnosticDecoderModule>();
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-  if (StaticPrefs::media_gmp_decoder_enabled() &&
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+  if (StaticPrefs::media_gmp_decoder_enabled() && !mFFmpegUsed &&
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       !CreateAndStartupPDM<GMPDecoderModule>()) {
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+    gUseKeyframeFromContainer = true;
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     mFailureFlags += DecoderDoctorDiagnostics::Flags::GMPPDMFailedToStartup;
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   }
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 }
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diff -up firefox-99.0/dom/media/platforms/PDMFactory.h.1670333 firefox-99.0/dom/media/platforms/PDMFactory.h
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--- firefox-99.0/dom/media/platforms/PDMFactory.h.1670333	2022-03-31 01:24:44.000000000 +0200
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+++ firefox-99.0/dom/media/platforms/PDMFactory.h	2022-04-04 09:58:35.606351546 +0200
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@@ -121,6 +121,7 @@ class PDMFactory final {
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   RefPtr<PlatformDecoderModule> mNullPDM;
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   DecoderDoctorDiagnostics::FlagsSet mFailureFlags;
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+  bool mFFmpegUsed = false;
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   friend class RemoteVideoDecoderParent;
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   static void EnsureInit();