diff --git a/mock/e2e/phy.spec.ts b/mock/e2e/phy.spec.ts
index 6a51259..acba6e2 100644
--- a/mock/e2e/phy.spec.ts
+++ b/mock/e2e/phy.spec.ts
@@ -728,7 +728,7 @@ test("スイッチ・スタビの十字はトグルで出し、スタビ候補
   const switchX = async () => (await mark.locator(".switch").boundingBox())!.x;
   const before = await switchX();
   await page.getByLabel("switchOffset X").fill("0.625");
-  await page.getByLabel("switchOffset X").blur();
+  await page.getByLabel("switchOffset X").dispatchEvent("change");
   await expect.poll(switchX).toBeGreaterThan(before + 10);
   await expect(mark.locator(".stab")).toHaveCount(4);
 
diff --git a/mock/src/domain/history.ts b/mock/src/domain/history.ts
index f50071d..6ee9509 100644
--- a/mock/src/domain/history.ts
+++ b/mock/src/domain/history.ts
@@ -10,12 +10,13 @@ export class ProjectHistory {
   ) {
     this.items = [structuredClone(initial)];
   }
-  /** Adds one undoable snapshot unless a grouped edit is active. */
+  /** Adds one undoable snapshot unless a grouped edit is active or nothing changed. */
   push(value: ProjectFile): void {
     if (this.pending) {
       this.replaceTop(value);
       return;
     }
+    if (JSON.stringify(value) === JSON.stringify(this.items[this.cursor])) return;
     this.add(value);
   }
   /** Starts grouping subsequent pushes into a single operation. T-17. */
diff --git a/mock/tests/domain.test.ts b/mock/tests/domain.test.ts
index 56e9033..801468a 100644
--- a/mock/tests/domain.test.ts
+++ b/mock/tests/domain.test.ts
@@ -149,6 +149,16 @@ describe("domain project flow", () => {
     expect(h.undo().phy.metadata.name).toBe(p.phy.metadata.name);
     expect(h.redo().phy.metadata.name).toBe("drag");
   });
+  it("does not add a history step for an unchanged snapshot", async () => {
+    const p = await createProjectFromKle(kle, { idGenerator: gen }),
+      h = new ProjectHistory(p);
+    const q = structuredClone(p);
+    q.phy.metadata.name = "edit";
+    h.push(q);
+    h.push(structuredClone(q));
+    expect(h.undo().phy.metadata.name).toBe(p.phy.metadata.name);
+    expect(h.canUndo).toBe(false);
+  });
   it("imports the VIA geometry sample", async () => {
     const via = JSON.parse(
       readFileSync(new URL("../../samples/via/cr0wn60.json", import.meta.url), "utf8"),