test,docs: fix section 6 vsink ref; update docs with SDL timing results and RenderPresent root cause

tests/test_video_playback_device.py

@@ -247,14 +247,20 @@ else:
     live_frames = 0
     live_error = None
 
-    LIVE_PIPE = (
-        f"playbin uri=\"{test_url}\" "
-        f"video-sink=\"videoconvert ! video/x-raw,format=BGRA ! appsink name=vsink emit-signals=true max-buffers=2 drop=true\""
-    )
-
     try:
-        pipe = Gst.parse_launch(LIVE_PIPE)
-        vsink = pipe.get_by_name("vsink")
+        # Build the pipeline element-by-element so we can hold a direct
+        # reference to the appsink (parse_launch embeds it in a bin and
+        # get_by_name returns None when the bin wraps a nested pipeline string).
+        pipe = Gst.ElementFactory.make("playbin", "live_player")
+        vsink = Gst.ElementFactory.make("appsink", "vsink")
+        if pipe is None or vsink is None:
+            raise RuntimeError("playbin or appsink not available")
+        vsink.set_property("emit-signals", True)
+        vsink.set_property("max-buffers", 2)
+        vsink.set_property("drop", True)
+        vsink.set_property("caps", Gst.Caps.from_string("video/x-raw,format=BGRA"))
+        pipe.set_property("video-sink", vsink)
+        pipe.set_property("uri", test_url if "://" in test_url else Gst.filename_to_uri(test_url))
 
         def _on_live_sample(sink, *_):
             global live_frames