fix: keep the displayed frame during frame stepping

Sources/Core/player.cpp

@@ -269,9 +269,9 @@ struct Player {
         check(avcodec_open2(track.codec, decoder, nullptr), "Opening decoder");
     }
 
-    void resetTimeline(double target) {
+    void resetTimeline(double target, bool clearVideo = true) {
         output.pause();
-        output.reset();
+        output.reset(clearVideo);
         for (auto &track : tracks) {
             if (!track.codec) continue;
             avcodec_flush_buffers(track.codec);
@@ -425,7 +425,8 @@ struct Player {
         output.pause();
         check(av_seek_frame(format, tracks[1].index, timestamp, AVSEEK_FLAG_BACKWARD),
               "Seeking video keyframe");
-        resetTimeline(target);
+        // Decoder preroll must not blank the last presented frame between steps.
+        resetTimeline(target, false);
         status = PLAYER_PAUSED;
     }