fix(core): the filter tier's svg no longer changes which child is last in the root

packages/core/src/runtime/captureMotionBlur/filterTier.ts

@@ -167,7 +167,7 @@ export function createFilterTier(
       defs.setAttribute("data-hf-ignore", "");
       defs.style.cssText = "position:absolute;left:0;top:0;pointer-events:none";
       node(defs, "defs", {});
-      root.append(defs);
+      document.documentElement.append(defs);
     }
     layer = buildLayer(defs.querySelector("defs") ?? defs, layers.size, el);
     layers.set(el, layer);

packages/core/src/runtime/captureMotionBlur/index.test.ts

@@ -2078,6 +2078,25 @@ describe("filter fallback", () => {
     expect(h.seek.mock.calls.every(([, suppress]) => suppress === true)).toBe(true);
   });
 
+  it("keeps the filter tier's svg out of the root and removes it on teardown", () => {
+    const { root, h } = movingHost();
+    const svgs = () => document.querySelectorAll("svg[data-hf-ignore]").length;
+    initCaptureMotionBlur(h);
+    void paintCaptureMotionBlur(0.5, PLAY);
+    expect(root.querySelector("svg[data-hf-ignore]")).toBeNull();
+    expect(svgs()).toBe(1);
+    initCaptureMotionBlur({ ...h, timeline: () => "tl2" });
+    void paintCaptureMotionBlur(0.6, PLAY);
+    expect(svgs()).toBe(1);
+    const next = movingHost();
+    initCaptureMotionBlur(next.h);
+    void paintCaptureMotionBlur(0.5, PLAY);
+    expect(next.root.querySelector("svg[data-hf-ignore]")).toBeNull();
+    expect(svgs()).toBe(1);
+    resetCaptureMotionBlurForTests();
+    expect(svgs()).toBe(0);
+  });
+
   it("removes its filters and its SVG on teardown", () => {
     const { el, h } = movingHost();
     initCaptureMotionBlur(h);

packages/engine/src/services/__fixtures__/motionBlur/thresholds.ts

@@ -58,7 +58,6 @@ export const SWEEP_SYMPTOMS = {
   F_CAPTURE: 40.62,
   F_FILTER: 126.54,
   H: 135.67,
-  I: 53.63,
   J: 25.79,
   K: MEASURED_ORACLE_SMALLEST_SYMPTOM,
   M: 18.31,

packages/engine/src/services/captureMotionBlur.sweep.browser.test.ts

@@ -184,12 +184,6 @@ const CASES: readonly SweepCase[] = [
     tier: "filter",
     frame: 11,
     check: "outside",
-    bug: {
-      finding: "I",
-      reason: "the filter tier's svg becomes the root's last child, so :last-child moves to it",
-      regions: [rect(0, 0, 0, 0), rect(494, 14, 112, 62)],
-      measuredMae: S.I,
-    },
   },
   {
     fixture: "overflow-card",