diff --git a/src/components/PlanEditor.test.ts b/src/components/PlanEditor.test.ts
new file mode 100644
index 0000000..fcfd294
--- /dev/null
+++ b/src/components/PlanEditor.test.ts
@@ -0,0 +1,73 @@
+// @vitest-environment happy-dom
+import { describe, it, expect, beforeAll } from "vitest"
+import { mount } from "@vue/test-utils"
+import PlanEditor from "./PlanEditor.vue"
+
+// happy-dom ships no canvas 2D context; measure() only needs measureText, so a
+// tiny stub keeps onMounted from throwing without pulling in a canvas polyfill.
+beforeAll(() => {
+  HTMLCanvasElement.prototype.getContext = (() => ({
+    measureText: () => ({ width: 80 }),
+    font: "",
+  })) as unknown as HTMLCanvasElement["getContext"]
+})
+
+/** Mount with a working v-model so emitted edits flow back into the prop. */
+function mountEditor(initial = "") {
+  const wrapper = mount(PlanEditor, {
+    props: {
+      modelValue: initial,
+      error: null,
+      "onUpdate:modelValue": (v: string) => wrapper.setProps({ modelValue: v }),
+    },
+  })
+  return wrapper
+}
+
+/** Type `text` into the textarea (caret at the end) and let the popup refresh. */
+async function type(wrapper: ReturnType<typeof mountEditor>, text: string) {
+  const ta = wrapper.find("textarea")
+  const el = ta.element as HTMLTextAreaElement
+  el.value = text
+  el.selectionStart = el.selectionEnd = text.length
+  await ta.trigger("input")
+}
+
+const lastEmit = (wrapper: ReturnType<typeof mountEditor>) => {
+  const events = wrapper.emitted("update:modelValue")
+  return events?.at(-1)?.[0] as string | undefined
+}
+
+describe("PlanEditor completion keys", () => {
+  it("opens the popup as the author types a key prefix", async () => {
+    const wrapper = mountEditor()
+    await type(wrapper, "t")
+    expect(wrapper.find(".completion").exists()).toBe(true)
+    expect(wrapper.findAll(".completion .label").map((el) => el.text())).toContain("title")
+  })
+
+  it("Tab accepts the selected item without navigating first", async () => {
+    const wrapper = mountEditor()
+    await type(wrapper, "t")
+    await wrapper.find("textarea").trigger("keydown", { key: "Tab" })
+    expect(lastEmit(wrapper)).toBe("title = ")
+    expect(wrapper.find(".completion").exists()).toBe(false)
+  })
+
+  it("Enter does not accept an untouched popup — it closes instead", async () => {
+    const wrapper = mountEditor()
+    await type(wrapper, "t")
+    await wrapper.find("textarea").trigger("keydown", { key: "Enter" })
+    // No accept happened: the last emit is still the raw typed value.
+    expect(lastEmit(wrapper)).toBe("t")
+    expect(wrapper.find(".completion").exists()).toBe(false)
+  })
+
+  it("Enter accepts once the author has navigated the popup", async () => {
+    const wrapper = mountEditor()
+    await type(wrapper, "t")
+    await wrapper.find("textarea").trigger("keydown", { key: "ArrowDown" })
+    await wrapper.find("textarea").trigger("keydown", { key: "Enter" })
+    expect(lastEmit(wrapper)).toBe("title = ")
+  })
+})