diff --git a/crates/opentake-render/src/gpu/compositor.rs b/crates/opentake-render/src/gpu/compositor.rs
index 9d9440c..7790111 100644
--- a/crates/opentake-render/src/gpu/compositor.rs
+++ b/crates/opentake-render/src/gpu/compositor.rs
@@ -358,8 +358,15 @@ impl Compositor {
                 affine1_nat: [
                     draw.affine[4] as f32,
                     draw.affine[5] as f32,
-                    tex.width as f32,
-                    tex.height as f32,
+                    // Source natural size the affine was built with — NOT the
+                    // decoded texture resolution. The preview decodes at a
+                    // downscaled max_size, so `tex.width/height` here mismatched
+                    // the affine's nat and rendered the layer shrunk into the
+                    // bottom-left corner, jittering as the texture size varied
+                    // (#125). UV (crop_uv) samples the texture 0..1, so its actual
+                    // pixel size is irrelevant to geometry.
+                    draw.nat_size.0 as f32,
+                    draw.nat_size.1 as f32,
                 ],
                 canvas_op_flags: [
                     size.width as f32,
diff --git a/crates/opentake-render/src/plan/build.rs b/crates/opentake-render/src/plan/build.rs
index de426fc..4684272 100644
--- a/crates/opentake-render/src/plan/build.rs
+++ b/crates/opentake-render/src/plan/build.rs
@@ -290,6 +290,10 @@ fn eval_layer<'a>(
         source: &plan.source,
         source_frame,
         affine,
+        // Carry the SAME natural size the affine was built with (above) so the
+        // shader's quad lands in the right place regardless of the (possibly
+        // downscaled) decoded texture resolution (#125).
+        nat_size: plan.nat_size,
         crop_uv,
         opacity,
         needs_premultiply: plan.needs_premultiply,
diff --git a/crates/opentake-render/src/plan/types.rs b/crates/opentake-render/src/plan/types.rs
index 2676134..4d6919b 100644
--- a/crates/opentake-render/src/plan/types.rs
+++ b/crates/opentake-render/src/plan/types.rs
@@ -134,6 +134,14 @@ pub struct LayerDraw<'a> {
     /// `[a, b, c, d, tx, ty]`; coordinate frame: origin bottom-left, y up, unit =
     /// pixels (SPEC §1.3 projection convention).
     pub affine: [f64; 6],
+    /// Source natural size (pixels) the `affine` was built against — the shader
+    /// scales its `[0,1]` quad by THIS to recover source-pixel space before
+    /// applying `affine`. It MUST be the value passed to `affine_transform`
+    /// (`ClipPlan.nat_size`), NOT the decoded texture's resolution: the preview
+    /// decodes at a downscaled `max_size`, so a texture-size proxy mismatches the
+    /// affine and renders the layer shrunk into a corner (and jittering as the
+    /// texture size varies). SPEC §1.3 / §3.3.
+    pub nat_size: (f64, f64),
     /// Source-texture UV sub-rect (folded from `crop_at(f)`), `(u0, v0, u1, v1)`
     /// in `[0, 1]`. SPEC §3.4.
     pub crop_uv: (f64, f64, f64, f64),
diff --git a/crates/opentake-render/tests/gpu_downscaled_nat.rs b/crates/opentake-render/tests/gpu_downscaled_nat.rs
new file mode 100644
index 0000000..4ee0e79
--- /dev/null
+++ b/crates/opentake-render/tests/gpu_downscaled_nat.rs
@@ -0,0 +1,112 @@
+//! Regression test for #125 — the preview decodes source frames at a downscaled
+//! `max_size`, so a layer's GPU texture is SMALLER than its source natural size.
+//!
+//! The shader scales its `[0,1]` quad by the SOURCE natural size (the value the
+//! affine was built with), NOT by the decoded texture's resolution. If the
+//! compositor feeds the texture size into the `nat` uniform instead, a full-canvas
+//! clip renders shrunk into the bottom-left corner (and jitters as the per-frame
+//! decoded size varies). This test composites a 4×4 texture for a 16×16 source on
+//! a 16×16 canvas and asserts the layer still fills the whole frame.
+//!
+//! HARD CONSTRAINT: skips gracefully (eprintln + early return) when no GPU adapter
+//! is available — never fails on GPU absence.
+
+use std::rc::Rc;
+
+use opentake_domain::{Clip, ClipType, Point, Timeline, Track, Transform};
+use opentake_render::gpu::texture::upload_rgba;
+use opentake_render::source::DecodedFrame;
+use opentake_render::wgpu;
+use opentake_render::{
+    build_render_plan, Compositor, GpuTexture, RenderDevice, RenderSize, SourceMetrics,
+    TextureResolver, TextureSource,
+};
+
+const RS: RenderSize = RenderSize {
+    width: 16,
+    height: 16,
+};
+
+/// Source display size is 16×16 (== canvas); the decoded texture will be 4×4.
+struct Metrics;
+impl SourceMetrics for Metrics {
+    fn natural_size(&self, _r: &str) -> Option<(u32, u32)> {
+        Some((16, 16))
+    }
+}
+
+/// Resolves to a 4×4 solid texture — a quarter of the source/canvas size per
+/// axis, exactly the downscale the preview applies (decode `max_size` < source).
+struct SmallTexResolver<'d> {
+    device: &'d wgpu::Device,
+    queue: &'d wgpu::Queue,
+    rgba: [u8; 4],
+}
+impl TextureResolver for SmallTexResolver<'_> {
+    fn resolve(&mut self, _s: &TextureSource, _f: i64) -> Option<Rc<GpuTexture>> {
+        let mut buf = vec![0u8; 4 * 4 * 4];
+        for px in buf.chunks_exact_mut(4) {
+            px.copy_from_slice(&self.rgba);
+        }
+        let frame = DecodedFrame::new(4, 4, buf, true); // premultiplied solid
+        Some(Rc::new(upload_rgba(
+            self.device,
+            self.queue,
+            &frame,
+            false,
+            Some("small"),
+        )))
+    }
+}
+
+#[test]
+fn downscaled_texture_full_canvas_still_fills_whole_frame() {
+    let dev = match RenderDevice::try_new() {
+        Ok(d) => d,
+        Err(e) => {
+            eprintln!(
+                "[skip] downscaled_texture_full_canvas_still_fills_whole_frame: no GPU ({e})"
+            );
+            return;
+        }
+    };
+
+    let mut tl = Timeline::new();
+    tl.fps = 30;
+    tl.width = 16;
+    tl.height = 16;
+    let mut clip = Clip::new("c0", "asset", 0, 10);
+    clip.transform = Transform::from_top_left(Point { x: 0.0, y: 0.0 }, 1.0, 1.0);
+    let mut track = Track::new("t0", ClipType::Video);
+    track.clips.push(clip);
+    tl.tracks.push(track);
+
+    let plan = build_render_plan(&tl, RS, &Metrics);
+    let fp = plan.frame(&tl, 0);
+    assert_eq!(fp.draws.len(), 1);
+    // The draw must carry the SOURCE natural size (16×16), independent of the 4×4
+    // decoded texture — this is the contract the fix restores.
+    assert_eq!(fp.draws[0].nat_size, (16.0, 16.0));
+
+    let compositor = Compositor::new(&dev.device);
+    let mut resolver = SmallTexResolver {
+        device: &dev.device,
+        queue: &dev.queue,
+        rgba: [40, 200, 80, 255],
+    };
+    let frame = compositor
+        .render_to_rgba(&dev.device, &dev.queue, RS, &fp, &mut resolver)
+        .expect("render");
+
+    // A full-canvas clip with a downscaled texture must fill EVERY pixel with the
+    // source color. Before the fix the content rendered only in the bottom-left
+    // 4×4 quarter (quad scaled by tex 4 instead of nat 16), leaving the rest
+    // opaque black — so any pixel being black catches the regression.
+    for (i, px) in frame.rgba.chunks_exact(4).enumerate() {
+        assert_eq!(
+            px,
+            &[40, 200, 80, 255],
+            "pixel {i} must be the source color — a full-canvas downscaled texture must fill the whole frame (#125)"
+        );
+    }
+}