fix: skip disposable pictures during local seek preroll

Sources/Core/player.cpp

@@ -468,6 +468,7 @@ struct Player {
         for (auto &track : tracks) {
             if (!track.codec) continue;
             avcodec_flush_buffers(track.codec);
+            track.codec->skip_frame = AVDISCARD_DEFAULT;
             track.drainSent = track.drained = false;
             track.firstPacket = true;
             av_packet_unref(track.packet);
@@ -1151,6 +1152,17 @@ struct Player {
                           "Preparing video decoder preroll");
                     track.firstPacket = false;
                 }
+                if (slot == 1) {
+                    // Preroll needs reference pictures, not disposable pictures before
+                    // the target. Restore full decoding before the target packet and
+                    // for exact frame stepping; never guess when timestamps are absent.
+                    const auto *packet = track.packet;
+                    const bool preroll = !network && !stepping && !previewShown && seekTarget > 0 &&
+                        packet->pts != AV_NOPTS_VALUE &&
+                        packet->duration > 0 &&
+                        (double(packet->pts) + packet->duration) * av_q2d(track.stream()->time_base) - origin < seekTarget;
+                    track.codec->skip_frame = preroll ? AVDISCARD_NONREF : AVDISCARD_DEFAULT;
+                }
                 const int result = avcodec_send_packet(track.codec, track.packet);
                 if (result != AVERROR(EAGAIN)) {
                     check(result, "Sending media packet to decoder");