render: stack higher-numbered video tracks on top

Match the timeline UI (V_max drawn topmost): composite tracks from
V1 up to V_max so the highest-numbered track is composited last, in
both the montage path and direct graph evaluation.

crates/oak-app/src/oakui/renderops.rs

@@ -175,9 +175,11 @@ fn clip_effects(g: &oak_node::graph::Graph, block_id: NodeId) -> Vec<oak_render:
 }
 
 /// The video montage at sequence time `time`: every clip covering `time`
-/// on video tracks, ordered bottom-to-top (track index 0 is topmost, so
-/// it is composited last). Hidden tracks (the muted flag doubles as the
-/// video visibility toggle, Olive parity) contribute nothing.
+/// on video tracks, ordered bottom-to-top (the highest-numbered track —
+/// the list's last — is topmost, so it is composited last; NLE stacking
+/// matches the timeline UI, which shows the highest-numbered track on
+/// top). Hidden tracks (the muted flag doubles as the video visibility
+/// toggle, Olive parity) contribute nothing.
 pub fn video_montage(p: &ProjectRef, seq: NodeId, time: Rational) -> Vec<MontageClip> {
 	let g = lock(p);
 	let mut clips = Vec::new();
@@ -191,7 +193,7 @@ pub fn video_montage(p: &ProjectRef, seq: NodeId, time: Rational) -> Vec<Montage
 		if list.kind != TrackType::Video {
 			continue;
 		}
-		for &track_id in list.tracks.iter().rev() {
+		for &track_id in list.tracks.iter() {
 			let Some(track) = track_behavior(&g.graph, track_id) else {
 				continue;
 			};
@@ -959,6 +961,73 @@ mod tests {
 		let _ = std::fs::remove_file(&media);
 	}
 
+	/// NLE stacking regression: two OPAQUE solid-color clips covering the
+	/// same time on V1 (red) and V2 (blue) — the montage lists V1's clip
+	/// first and V2's LAST (the topmost composites last), and the rendered
+	/// frame shows V2's blue (the highest-numbered track wins, matching
+	/// the timeline UI).
+	#[test]
+	fn video_montage_stacks_highest_track_on_top() {
+		let _media = media_lock();
+		oak_undo::global::clear().unwrap();
+		let red =
+			std::env::temp_dir().join(format!("oakapp_stack_red_{}.mp4", std::process::id()));
+		let blue =
+			std::env::temp_dir().join(format!("oakapp_stack_blue_{}.mp4", std::process::id()));
+		oak_codec::testmedia::write_test_clip_solid(&red, 64, 64, 10, 10, [0.9, 0.1, 0.1, 1.0])
+			.expect("generate the red media");
+		oak_codec::testmedia::write_test_clip_solid(&blue, 64, 64, 10, 10, [0.1, 0.1, 0.9, 1.0])
+			.expect("generate the blue media");
+
+		let project = graphops::create_project();
+		let seq = graphops::create_sequence(&project, "Stack Montage");
+		let red_footage = graphops::import_footage(&project, &red).expect("import the red media");
+		let blue_footage = graphops::import_footage(&project, &blue).expect("import the blue media");
+		graphops::place_footage_clip(&project, seq, red_footage, TrackType::Video, 0, 0, 10, 0)
+			.expect("place the V1 (red) clip");
+		graphops::place_footage_clip(&project, seq, blue_footage, TrackType::Video, 1, 0, 10, 0)
+			.expect("place the V2 (blue) clip");
+		let tb = graphops::sequence_time_base(&lock(&project).graph, seq).unwrap();
+		let time = graphops::ts_to_rational(0, tb);
+
+		// Bottom-to-top: V1's (red) clip first, V2's (blue) last.
+		let montage = video_montage(&project, seq, time);
+		assert_eq!(montage.len(), 2, "both clips cover frame 0");
+		assert_eq!(montage[0].filename, red.to_string_lossy(), "V1's clip is the bottom of the stack");
+		assert_eq!(
+			montage[1].filename,
+			blue.to_string_lossy(),
+			"V2's clip is the top of the stack (composited last)"
+		);
+
+		// Render through the same entry point the render worker uses: V2's
+		// blue covers V1's red.
+		let params = VideoTicketParams {
+			viewer: 0,
+			project: String::new(),
+			time,
+			force_size: Some((64, 64)),
+			force_format: Some(oak_core::PixelFormat::F32),
+			cache: None,
+			cache_dir: None,
+			cache_id: None,
+			cache_timebase: None,
+			footage: None,
+			montage,
+		};
+		let mut dst = vec![0u8; 64 * 64 * 16];
+		oak_render::eval::render_montage_frame_into(time, &params, (64, 64), &mut dst, 64 * 16)
+			.expect("montage render");
+		let off = (8 * 64 + 8) * 16;
+		let r = f32::from_le_bytes(dst[off..off + 4].try_into().unwrap());
+		let b = f32::from_le_bytes(dst[off + 8..off + 12].try_into().unwrap());
+		assert!(b > 0.5 && r < 0.4, "V2's blue covers V1's red (r={r}, b={b})");
+
+		oak_undo::global::clear().unwrap();
+		let _ = std::fs::remove_file(&red);
+		let _ = std::fs::remove_file(&blue);
+	}
+
 	/// The acceptance gate for montage effect rendering: a 50% Opacity on
 	/// a real clip (generated media, real graph, real decode, real
 	/// compositing — nothing mocked) must change the rendered pixels, and

crates/oak-cli/src/engine.rs

@@ -492,8 +492,8 @@ fn clip_media(g: &Graph, block_id: NodeId) -> Option<(String, i32)> {
 }
 
 /// The video montage at sequence time `time`: every clip covering `time`
-/// on video tracks, ordered bottom-to-top (track index 0 is topmost, so
-/// it is composited last).
+/// on video tracks, ordered bottom-to-top (the highest-numbered track —
+/// the list's last — is topmost, so it is composited last).
 pub fn video_montage(p: &ProjectRef, seq_id: NodeId, time: Rational) -> Vec<MontageClip> {
 	let g = lock(p);
 	let mut clips = Vec::new();
@@ -507,7 +507,7 @@ pub fn video_montage(p: &ProjectRef, seq_id: NodeId, time: Rational) -> Vec<Mont
 		if list.kind != TrackType::Video {
 			continue;
 		}
-		for &track_id in list.tracks.iter().rev() {
+		for &track_id in list.tracks.iter() {
 			let Some(track) = track_behavior(&g.graph, track_id) else {
 				continue;
 			};

crates/oak-codec/src/testmedia.rs

@@ -58,23 +58,7 @@ pub fn write_test_clip(
 	if width <= 0 || height <= 0 || frame_count <= 0 || fps <= 0 {
 		return Err(Error::Invalid);
 	}
-	let mut params = EncodingParams::default();
-	let name = out.as_os_str().as_encoded_bytes();
-	if name.len() >= params.filename.len() {
-		return Err(Error::Failed("output path too long".into()));
-	}
-	params.filename[..name.len()].copy_from_slice(name);
-	params.format = 2; // MPEG-4 video container
-	params.video_enabled = 1;
-	params.video_codec = 10; // MPEG-2 (B-frame-free; the H.264 B-frame streams hit a muxer timing bug)
-	params.video_width = width;
-	params.video_height = height;
-	params.video_time_base_num = 1;
-	params.video_time_base_den = fps;
-	params.video_pixel_format = PixelFormat::F32;
-	params.video_interlacing = 0;
-	params.video_pixel_aspect_num = 1;
-	params.video_pixel_aspect_den = 1;
+	let mut params = video_params(out, width, height, fps)?;
 	// A stereo PCM audio track with a known 440 Hz sine (M12 P1: the
 	// audio-render path needs a decodable audio stream; PCM avoids
 	// codec sample-format negotiation issues in this FFmpeg pairing).
@@ -103,6 +87,75 @@ pub fn write_test_clip(
 	encoder.flush()
 }
 
+/// Encode `frame_count` frames of a solid `rgba` color into `out` (same
+/// MPEG-2/MP4 pairing as [`write_test_clip`], video only). The stacking
+/// tests need two OPAQUE clips of different known colors covering the
+/// same time — the sweeping pattern cannot provide that (every clip's
+/// frame 0 is identical, and decode-time seeking is not exercised here).
+pub fn write_test_clip_solid(
+	out: &Path,
+	width: i32,
+	height: i32,
+	frame_count: i32,
+	fps: i32,
+	rgba: [f32; 4],
+) -> Result<()> {
+	if width <= 0 || height <= 0 || frame_count <= 0 || fps <= 0 {
+		return Err(Error::Invalid);
+	}
+	let params = video_params(out, width, height, fps)?;
+	let encoder = create_from_params(&params)
+		.ok_or_else(|| Error::Failed("no encoder for test clip params".into()))?;
+	encoder.configure(&params)?;
+	encoder.open()?;
+	for i in 0..frame_count {
+		encoder.write_video(&solid_frame(i, width, height, fps, rgba))?;
+	}
+	encoder.flush()
+}
+
+/// The shared MPEG-2-video-in-MP4 encoder params of the test clips
+/// (B-frame-free; the H.264 B-frame streams hit a muxer timing bug).
+fn video_params(out: &Path, width: i32, height: i32, fps: i32) -> Result<EncodingParams> {
+	let mut params = EncodingParams::default();
+	let name = out.as_os_str().as_encoded_bytes();
+	if name.len() >= params.filename.len() {
+		return Err(Error::Failed("output path too long".into()));
+	}
+	params.filename[..name.len()].copy_from_slice(name);
+	params.format = 2; // MPEG-4 video container
+	params.video_enabled = 1;
+	params.video_codec = 10; // MPEG-2 (B-frame-free; the H.264 B-frame streams hit a muxer timing bug)
+	params.video_width = width;
+	params.video_height = height;
+	params.video_time_base_num = 1;
+	params.video_time_base_den = fps;
+	params.video_pixel_format = PixelFormat::F32;
+	params.video_interlacing = 0;
+	params.video_pixel_aspect_num = 1;
+	params.video_pixel_aspect_den = 1;
+	Ok(params)
+}
+
+/// One frame of solid `rgba` (F32 RGBA rows, like [`pattern_frame`]).
+fn solid_frame(i: i32, width: i32, height: i32, fps: i32, rgba: [f32; 4]) -> Frame {
+	let mut f = pattern_frame(0, width, height, fps);
+	f.set_timestamp(Rational::new(i as i64, fps as i64));
+	let linesize = f.linesize_bytes() as usize;
+	let data = f.data_mut().expect("test frame buffer");
+	let [r, g, b, a] = rgba;
+	for y in 0..height as usize {
+		for x in 0..width as usize {
+			let off = y * linesize + x * 16;
+			data[off..off + 4].copy_from_slice(&r.to_le_bytes());
+			data[off + 4..off + 8].copy_from_slice(&g.to_le_bytes());
+			data[off + 8..off + 12].copy_from_slice(&b.to_le_bytes());
+			data[off + 12..off + 16].copy_from_slice(&a.to_le_bytes());
+		}
+	}
+	f
+}
+
 /// One frame of the known pattern (see module doc).
 fn pattern_frame(i: i32, width: i32, height: i32, fps: i32) -> Frame {
 	let mut vp = VideoParams::new_basic(

crates/oak-render/src/eval.rs

@@ -1008,8 +1008,9 @@ fn composite_tracks(frames: Vec<Frame>, size: (i32, i32)) -> Frame {
 /// Render one frame of `viewer` (a sequence) at `time`: evaluate every
 /// enabled clip overlapping `time` through the node graph (one traverser
 /// pass per clip; the hooks' decoder cache is shared across clips) and
-/// composite the resulting frames topmost-first — in the video track
-/// list, track 0 is the topmost stack element (C++ `TrackList` order).
+/// composite the resulting frames bottommost-first — in the video track
+/// list the LAST track (the highest-numbered one) is the topmost stack
+/// element (NLE stacking, matching the timeline UI).
 ///
 /// `size` is the decode target for every clip, so all frames composite
 /// without per-frame scaling. Errors: `Invalid` for a non-F32 format or a
@@ -1038,8 +1039,9 @@ pub fn render_graph_frame(
 
 	// Collect the clips covering `time`: the sequence's track lists (video
 	// then audio — C++ `Sequence` keeps them in the `k_track_input_format`
-	// array order), the video list's tracks in stack order, then each
-	// track's blocks.
+	// array order), the video list's tracks topmost-first (the list's last
+	// track is the top of the stack; `composite_tracks` walks the frames in
+	// reverse and draws the first frame last), then each track's blocks.
 	let mut clips: Vec<oak_node::id::NodeId> = Vec::new();
 	for tl_id in &sequence.track_lists {
 		let Some(tl) = graph.get(*tl_id) else {
@@ -1055,7 +1057,7 @@ pub fn render_graph_frame(
 		if tl.kind != oak_node::track::TrackType::Video {
 			continue;
 		}
-		for track_id in &tl.tracks {
+		for track_id in tl.tracks.iter().rev() {
 			let Some(track) = graph.get(*track_id) else {
 				continue;
 			};
@@ -1297,7 +1299,8 @@ fn scale_rgba_f32(
 }
 
 /// Composite the montage at `time`: decode each covering clip and
-/// alpha-composite topmost-last (C++ track order: track 0 is topmost).
+/// alpha-composite topmost-last (NLE track order: the highest-numbered
+/// track is topmost).
 fn render_montage_frame(
 	time: Rational,
 	params: &crate::ticket::VideoTicketParams,

crates/oak-render/tests/graph_render.rs

@@ -19,7 +19,8 @@
 //! Builds a real node graph (sequence -> video track list -> video tracks
 //! -> clip blocks -> footage), evaluates the clip overlapping the request
 //! time through the traverser and composites the decoded frames — the same
-//! path the engine's viewer uses. Track 0 is the topmost stack element.
+//! path the engine's viewer uses. The LAST track is the topmost stack
+//! element (NLE stacking: the highest-numbered track wins).
 
 use std::sync::{Arc, Mutex};
 
@@ -43,7 +44,8 @@ fn clip_path(tag: &str) -> std::path::PathBuf {
 }
 
 /// One sequence + one video track list with one track per clip
-/// `(filename, [in, out))`. Track 0 (the first entry) composites on top.
+/// `(filename, [in, out))`. The LAST entry's track composites on top
+/// (NLE stacking: the highest-numbered track is topmost).
 fn build_project(clips: &[(&str, Rational, Rational)]) -> (Arc<Mutex<Project>>, NodeId) {
     let project = Project::new();
     let seq;
@@ -191,6 +193,51 @@ fn graph_sequence_renders_two_tracks() {
     let _ = std::fs::remove_file(&path_b);
 }
 
+/// NLE stacking regression: two OPAQUE solid-color clips covering the
+/// same time on two video tracks — the clip on the LAST track (V2, blue)
+/// composites on top of the clip on the first track (V1, red), matching
+/// the timeline UI (the highest-numbered track displays on top).
+#[test]
+fn graph_sequence_stacks_highest_track_on_top() {
+    let red = clip_path("stack_red");
+    let blue = clip_path("stack_blue");
+    oak_codec::testmedia::write_test_clip_solid(&red, 64, 64, 10, 10, [0.9, 0.1, 0.1, 1.0])
+        .expect("red clip generation");
+    oak_codec::testmedia::write_test_clip_solid(&blue, 64, 64, 10, 10, [0.1, 0.1, 0.9, 1.0])
+        .expect("blue clip generation");
+
+    // V1 = red (bottom), V2 = blue (top).
+    let (project, seq) = build_project(&[
+        (&red.to_string_lossy(), Rational::new(0, 1), Rational::new(1, 1)),
+        (&blue.to_string_lossy(), Rational::new(0, 1), Rational::new(1, 1)),
+    ]);
+    let tex = oak_render::eval::render_graph_frame(&project, seq, Rational::new(0, 1), (64, 64), PixelFormat::F32)
+        .expect("stacked render");
+    let data = frame_data(&tex);
+    assert!(
+        channel(data, 8, 8, 2) > 0.5 && channel(data, 8, 8, 0) < 0.4,
+        "V2's blue covers V1's red (r={}, b={})",
+        channel(data, 8, 8, 0),
+        channel(data, 8, 8, 2)
+    );
+
+    // Distinguishability guard: solo, the V1 clip really is red (the two
+    // tracks carry different content).
+    let (solo, solo_seq) = build_project(&[(&red.to_string_lossy(), Rational::new(0, 1), Rational::new(1, 1))]);
+    let solo_tex = oak_render::eval::render_graph_frame(&solo, solo_seq, Rational::new(0, 1), (64, 64), PixelFormat::F32)
+        .expect("solo V1 render");
+    let solo_data = frame_data(&solo_tex);
+    assert!(
+        channel(solo_data, 8, 8, 0) > 0.5 && channel(solo_data, 8, 8, 2) < 0.4,
+        "solo V1 is red (r={}, b={})",
+        channel(solo_data, 8, 8, 0),
+        channel(solo_data, 8, 8, 2)
+    );
+
+    let _ = std::fs::remove_file(&red);
+    let _ = std::fs::remove_file(&blue);
+}
+
 /// The driver rejects bad arguments explainably: non-F32 format, a
 /// non-positive size, and a missing viewer.
 #[test]

crates/oak-task/src/render.rs

@@ -330,7 +330,8 @@ impl RenderTask {
 	}
 
 	/// Flatten the video tracks of `sequence` into an ordered montage
-	/// (bottom-most track first so the topmost track composites last;
+	/// (bottom-most track first so the topmost track — the highest-numbered
+	/// one, the list's last — composites last;
 	/// `// CPP-PARITY: M12 P0 montage contract`).
 	fn video_montage(project: &ProjectRef, sequence: oak_node::id::NodeId, time: Rational) -> Vec<MontageClip> {
 		let guard = project
@@ -361,7 +362,7 @@ impl RenderTask {
 			if list.kind != TrackType::Video {
 				continue;
 			}
-			for track_id in list.tracks.iter().rev() {
+			for track_id in list.tracks.iter() {
 				let Some(te) = guard.graph.get(*track_id) else {
 					continue;
 				};