fix: keep the displayed frame during frame stepping

Sources/Core/output.hpp

@@ -21,8 +21,8 @@ public:
     // Atomic target gain; callback slews to it within 240 frames (5 ms).
     void set_volume(float normalized0to1) noexcept;
     float volume() const noexcept;
-    // Called with audio stopped. Clears audio and video from the old timeline.
-    void reset();
+    // Called with audio stopped. Clears audio; optionally keeps the displayed frame.
+    void reset(bool clear_video = true);
     std::size_t write(const float *samples, std::size_t frames, double pts);
     std::size_t queued() const;
     // Latency-aware presented position; NaN until an audio callback has run.

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;
     }