player/loadfile: possibly do a reset of sd_ass when turning on video

player/loadfile.c

@@ -745,6 +745,20 @@ void mp_switch_track_n(struct MPContext *mpctx, int order, enum stream_type type
         reselect_demux_stream(mpctx, current, false);
     }
 
+    // For the edge case of switching from no video to a still image, we need to
+    // flush the ass events of any sub track for potential animations.
+    if (type == STREAM_VIDEO && order == 0 && !current && track->image) {
+        for (int n = 0; n < num_ptracks[STREAM_SUB]; n++) {
+            struct track *sub_track = mpctx->current_track[n][STREAM_SUB];
+            if (sub_track && sub_track->d_sub) {
+                sub_control(sub_track->d_sub, SD_CTRL_RESET_SOFT, NULL);
+                sub_reset(sub_track->d_sub);
+                if (sub_track->selected)
+                    reselect_demux_stream(mpctx, sub_track, true);
+            }
+        }
+    }
+
     mpctx->current_track[order][type] = track;
 
     if (track) {

sub/dec_sub.h

@@ -19,6 +19,7 @@ enum sd_ctrl {
     SD_CTRL_SET_ANIMATED_CHECK,
     SD_CTRL_SET_VIDEO_PARAMS,
     SD_CTRL_SET_VIDEO_DEF_FPS,
+    SD_CTRL_RESET_SOFT,
     SD_CTRL_UPDATE_OPTS,
 };
 

sub/sd_ass.c

@@ -998,6 +998,7 @@ static void reset(struct sd *sd)
     if (sd->opts->sub_clear_on_seek || ctx->clear_once) {
         ass_flush_events(ctx->ass_track);
         ctx->num_seen_packets = 0;
+        ctx->num_packets_animated = 0;
         sd->preload_ok = false;
         ctx->clear_once = false;
     }
@@ -1033,6 +1034,10 @@ static int control(struct sd *sd, enum sd_ctrl cmd, void *arg)
     case SD_CTRL_SET_ANIMATED_CHECK:
         ctx->check_animated = *(bool *)arg;
         return CONTROL_OK;
+    case SD_CTRL_RESET_SOFT:
+        ctx->clear_once = true;
+        reset(sd);
+        return CONTROL_OK;
     case SD_CTRL_SET_VIDEO_PARAMS:
         ctx->video_params = *(struct mp_image_params *)arg;
         return CONTROL_OK;