diff --git a/bucket_catch/packages/frontend/src/components/DropZone.test.tsx b/bucket_catch/packages/frontend/src/components/DropZone.test.tsx
index 2ea613fb..d6e31e6b 100644
--- a/bucket_catch/packages/frontend/src/components/DropZone.test.tsx
+++ b/bucket_catch/packages/frontend/src/components/DropZone.test.tsx
@@ -1,31 +1,16 @@
 import { describe, it, expect, vi } from "vitest";
 import { render, screen, fireEvent } from "@testing-library/react";
 import { DropZone } from "./DropZone";
+import { must } from "../test/domQueries";
 
 const makeFile = (name: string, type = "text/plain"): File =>
   new File(["x"], name, { type });
 
-/**
- * Query one element and narrow it, failing the test if it is absent.
- *
- * Both `as HTMLElement` and `!` are rejected here - the former by
- * non-nullable-type-assertion-style, the latter by no-non-null-assertion - so
- * the narrowing is done with a real check that also gives a useful message.
- */
-const must = (el: Element | null, what: string): HTMLElement => {
-  if (!(el instanceof HTMLElement)) {
-    throw new Error(`expected to find ${what}`);
-  }
-  return el;
-};
-
 describe("DropZone", () => {
   it("renders title and subtitle", () => {
     render(<DropZone onFiles={() => undefined} onPuzzle={() => undefined} />);
     expect(screen.getByText("Bucket Catch")).toBeInTheDocument();
-    expect(
-      screen.getByText(/Drop your files here/),
-    ).toBeInTheDocument();
+    expect(screen.getByText(/Drop your files here/)).toBeInTheDocument();
   });
 
   it("shows initial grid equation", () => {
@@ -37,10 +22,7 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={() => undefined} onPuzzle={() => undefined} />,
     );
-    const zone = must(
-      container.firstChild as Element | null,
-      "the drop zone",
-    );
+    const zone = must(container.firstChild as Element | null, "the drop zone");
     expect(zone.className).not.toContain("dragging");
     fireEvent.dragOver(zone);
     expect(zone.className).toContain("dragging");
@@ -53,10 +35,7 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={onFiles} onPuzzle={() => undefined} />,
     );
-    const zone = must(
-      container.firstChild as Element | null,
-      "the drop zone",
-    );
+    const zone = must(container.firstChild as Element | null, "the drop zone");
     const f1 = makeFile("a.txt");
     const f2 = makeFile("b.txt");
     fireEvent.drop(zone, { dataTransfer: { files: [f1, f2] } });
@@ -68,10 +47,7 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={() => undefined} onPuzzle={() => undefined} />,
     );
-    const zone = must(
-      container.firstChild as Element | null,
-      "the drop zone",
-    );
+    const zone = must(container.firstChild as Element | null, "the drop zone");
     fireEvent.drop(zone, { dataTransfer: { files: [makeFile("a.txt")] } });
     expect(screen.getByText("1 file ready")).toBeInTheDocument();
   });
@@ -81,10 +57,7 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={onFiles} onPuzzle={() => undefined} />,
     );
-    const zone = must(
-      container.firstChild as Element | null,
-      "the drop zone",
-    );
+    const zone = must(container.firstChild as Element | null, "the drop zone");
     fireEvent.drop(zone, { dataTransfer: { files: [] } });
     expect(onFiles).not.toHaveBeenCalled();
   });
@@ -201,9 +174,9 @@ describe("DropZone", () => {
       container.querySelector("input[accept='image/*']"),
       "input[accept='image/*']",
     );
-    const clickSpy = vi.spyOn(puzzleInput, "click").mockImplementation(
-      () => undefined,
-    );
+    const clickSpy = vi
+      .spyOn(puzzleInput, "click")
+      .mockImplementation(() => undefined);
     fireEvent.click(screen.getByText(/Play Puzzle Mode/));
     expect(clickSpy).toHaveBeenCalled();
   });
diff --git a/bucket_catch/packages/frontend/src/components/PuzzleResult.test.tsx b/bucket_catch/packages/frontend/src/components/PuzzleResult.test.tsx
index 7b8a44fd..7c24dfdd 100644
--- a/bucket_catch/packages/frontend/src/components/PuzzleResult.test.tsx
+++ b/bucket_catch/packages/frontend/src/components/PuzzleResult.test.tsx
@@ -1,6 +1,7 @@
 import { describe, it, expect, vi } from "vitest";
 import { render, screen, fireEvent } from "@testing-library/react";
 import { PuzzleResult } from "./PuzzleResult";
+import { must } from "../test/domQueries";
 import type { FallingPuzzleItem, PuzzlePiece } from "../types";
 
 const makePiece = (row: number, col: number): PuzzlePiece => ({
@@ -129,10 +130,10 @@ describe("PuzzleResult", () => {
     const imgs = container.querySelectorAll("img");
     expect(imgs).toHaveLength(2);
     // The grid div has inline style with gridTemplateColumns; its non-img children are holes
-    const gridEl = container.querySelector('[style*="repeat"]');
-    if (!(gridEl instanceof HTMLElement)) {
-      throw new Error("expected to find the puzzle grid element");
-    }
+    const gridEl = must(
+      container.querySelector('[style*="repeat"]'),
+      "the puzzle grid element",
+    );
     const holeDivs = Array.from(gridEl.children).filter(
       (c) => c.tagName === "DIV",
     );
diff --git a/bucket_catch/packages/frontend/src/test/domQueries.ts b/bucket_catch/packages/frontend/src/test/domQueries.ts
new file mode 100644
index 00000000..ab2a96f8
--- /dev/null
+++ b/bucket_catch/packages/frontend/src/test/domQueries.ts
@@ -0,0 +1,21 @@
+/**
+ * DOM query helpers shared by the component tests.
+ *
+ * Lives under src/test/ because the coverage config excludes that path — a
+ * helper placed beside a component would be counted as production code and
+ * break the 100% gate.
+ */
+
+/**
+ * Query one element and narrow it, failing the test if it is absent.
+ *
+ * Both `as HTMLElement` and `!` are rejected here — the former by
+ * non-nullable-type-assertion-style, the latter by no-non-null-assertion — so
+ * the narrowing is done with a real check that also gives a useful message.
+ */
+export const must = (el: Element | null, what: string): HTMLElement => {
+  if (!(el instanceof HTMLElement)) {
+    throw new Error(`expected to find ${what}`);
+  }
+  return el;
+};
diff --git a/docs/superpowers/evidence/bucket-catch-shared-dom-helper-2026-08-17.json b/docs/superpowers/evidence/bucket-catch-shared-dom-helper-2026-08-17.json
new file mode 100644
index 00000000..e6878158
--- /dev/null
+++ b/docs/superpowers/evidence/bucket-catch-shared-dom-helper-2026-08-17.json
@@ -0,0 +1,37 @@
+{
+  "intent": "Move the must() DOM narrowing helper out of DropZone.test.tsx, which the eslint pass had pushed 6 lines over the 250-line cap, into the shared coverage-excluded test directory where both component tests can use it.",
+  "scope": [
+    "New bucket_catch/packages/frontend/src/test/domQueries.ts",
+    "DropZone.test.tsx 256 -> 243 lines; PuzzleResult.test.tsx switched to the shared helper",
+    "Non-goal: no behaviour change, no assertion weakened"
+  ],
+  "changes": [
+    "Extracted must() into src/test/domQueries.ts. That directory is excluded by the coverage config, which is why fixtures already live there - a helper beside a component would be counted as production code and break the 100% gate.",
+    "DropZone.test.tsx imports it instead of defining it, dropping the file back under the cap.",
+    "PuzzleResult.test.tsx replaced its inline instanceof guard with the same helper, so the two component tests narrow DOM queries identically rather than each rolling their own."
+  ],
+  "verification": [
+    {
+      "command": "bash ~/utils/scripts/check_file_length.sh --all",
+      "result": "pass",
+      "evidence": "40 over-cap files before this change, 39 after; DropZone.test.tsx no longer appears. It had been 250 exactly and the must() helper pushed it to 256, so the eslint commit had silently added an over-cap file while removing others - this restores the intended 41 -> 39 for the session."
+    },
+    {
+      "command": "npm run coverage",
+      "result": "pass",
+      "evidence": "100% on all four measures with totals unchanged at 579 statements / 217 branches / 98 functions / 530 lines, and 145 tests in 15 files. Identical totals are the check that moving the helper into a coverage-excluded directory neither removed code from test reach nor added any."
+    },
+    {
+      "command": "npx tsc --noEmit && npx eslint src",
+      "result": "pass",
+      "evidence": "tsc exit 0; eslint still reports exactly the 4 known remaining errors, so the move introduced none."
+    }
+  ],
+  "risks": [
+    "src/test/domQueries.ts is not coverage-measured, so a bug in must() would not be caught by the coverage gate. It is four lines with a single instanceof check, and every caller exercises it on the happy path in tests that assert on the returned element."
+  ],
+  "rollback": [
+    "git revert the commit. must() returns to DropZone.test.tsx and that file goes back over the cap at 256 lines; PuzzleResult.test.tsx regains its inline guard.",
+    "After rollback, confirm npm run coverage still reports 100% with 579/217/98/530 and 145 passing tests."
+  ]
+}