fix(shader-transitions): pin cloned scene roots to composition dimensions

packages/shader-transitions/src/engineModePageComposite.test.ts

@@ -1,5 +1,6 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
 import {
+  clonePinStyleFor,
   isPageSideCompositingSupported,
   PAGE_COMPOSITOR_BUILD_CANARY,
   PAGE_COMPOSITOR_CANVAS_ID,
@@ -69,6 +70,26 @@ describe("isPageSideCompositingSupported", () => {
   });
 });
 
+describe("clonePinStyleFor", () => {
+  it("fixes a 0x0 inset:0 scene root to its live-measured box (the collapse this exists to prevent)", () => {
+    // A scene root sized only by `position:absolute; inset:0` measures as
+    // the full composition frame in the live document (its containing block
+    // there is the real ancestor chain) — collapses to 0x0 only once cloned
+    // into the staging canvas's own layout subtree.
+    const pin = clonePinStyleFor({ left: 0, top: 0, width: 1080, height: 1920 });
+    expect(pin).toEqual({ left: "0px", top: "0px", width: "1080px", height: "1920px" });
+  });
+
+  it("preserves an authored explicit width/height and offset instead of overriding it", () => {
+    // A scene root with its own explicit size/position (e.g. a picture-in-
+    // picture panel) measures as that exact box in the live document —
+    // clonePinStyleFor must reproduce it verbatim, not the full composition
+    // frame, or the clone would silently grow to fill the canvas.
+    const pin = clonePinStyleFor({ left: 120, top: 240, width: 400, height: 300 });
+    expect(pin).toEqual({ left: "120px", top: "240px", width: "400px", height: "300px" });
+  });
+});
+
 describe("page-side compositor exported constants", () => {
   it("exports a stable canary string used by the bundled-CLI smoke", () => {
     expect(PAGE_COMPOSITOR_BUILD_CANARY).toBe("__hf_page_compositor_v1__");

packages/shader-transitions/src/engineModePageComposite.ts

@@ -76,6 +76,36 @@ interface ResolvedTransition {
 export const PAGE_COMPOSITOR_CANVAS_ID = "__hf-page-side-compositor";
 export const PAGE_COMPOSITOR_BUILD_CANARY = "__hf_page_compositor_v1__";
 
+export interface ClonePinStyle {
+  left: string;
+  top: string;
+  width: string;
+  height: string;
+}
+
+/**
+ * Style values to pin a cloned scene root to the box its source measured
+ * WHILE STILL LIVE in the document — never the composition's full pixel size.
+ * A live-document `getBoundingClientRect()` already resolves `inset:0` (and
+ * any authored explicit width/height) correctly against the real ancestor
+ * chain; reapplying that exact box to the clone fixes the 0x0 collapse a
+ * detached `inset:0` clone would otherwise have inside the staging canvas's
+ * layout subtree, without ever overriding an author's own sizing.
+ */
+export function clonePinStyleFor(rect: {
+  left: number;
+  top: number;
+  width: number;
+  height: number;
+}): ClonePinStyle {
+  return {
+    left: `${rect.left}px`,
+    top: `${rect.top}px`,
+    width: `${rect.width}px`,
+    height: `${rect.height}px`,
+  };
+}
+
 export function isPageSideCompositingSupported(): boolean {
   if (typeof window === "undefined" || typeof document === "undefined") return false;
   if (!isHtmlInCanvasCaptureSupported()) return false;
@@ -232,6 +262,17 @@ export function installPageSideCompositor(options: PageCompositorInstallOptions)
       pWin.__hf_page_composite_pending = false;
       return false;
     }
+    // Measure each scene's rendered box WHILE STILL LIVE — a scene root sized
+    // only by `position:absolute; inset:0` resolves to 0x0 once cloned into
+    // the staging canvas's layout subtree (no containing-block dimensions
+    // there), and the transition textures blank out (wild report: explicit
+    // 1080x1920 anchors fixed both transitions). The live document already
+    // resolves inset:0 (and any authored explicit width/height) correctly
+    // against the real ancestor chain, so pinning the clone to THIS measured
+    // box fixes the collapse without ever overriding an author's own sizing.
+    const fromPin = clonePinStyleFor(fromEl.getBoundingClientRect());
+    const toPin = clonePinStyleFor(toEl.getBoundingClientRect());
+
     while (fromStaging.firstChild) fromStaging.removeChild(fromStaging.firstChild);
     while (toStaging.firstChild) toStaging.removeChild(toStaging.firstChild);
     const fromClone = fromEl.cloneNode(true) as HTMLElement;
@@ -244,9 +285,17 @@ export function installPageSideCompositor(options: PageCompositorInstallOptions)
     // paint record" and the shader degrades to a hard cut. The shader blends from
     // full-opacity textures via u_progress, so force the clones visible. Cf.
     // forceSceneVisibleInClone (html2canvas path).
-    for (const clone of [fromClone, toClone]) {
+    for (const [clone, pin] of [
+      [fromClone, fromPin],
+      [toClone, toPin],
+    ] as const) {
       clone.style.opacity = "1";
       clone.style.visibility = "visible";
+      clone.style.position = "absolute";
+      clone.style.left = pin.left;
+      clone.style.top = pin.top;
+      clone.style.width = pin.width;
+      clone.style.height = pin.height;
       clone.querySelectorAll<HTMLElement>("[data-start]").forEach((el) => {
         el.style.opacity = "1";
         el.style.visibility = "visible";