diff --git a/frontend/__tests__/elements/test-activity.spec.ts b/frontend/__tests__/elements/test-activity.spec.ts
new file mode 100644
index 000000000000..96801c51f56c
--- /dev/null
+++ b/frontend/__tests__/elements/test-activity.spec.ts
@@ -0,0 +1,28 @@
+import { describe, expect, it } from "vitest";
+
+import { getActivityBalloonPos } from "../../src/ts/elements/test-activity";
+
+describe("test-activity.ts", () => {
+  describe("getActivityBalloonPos", () => {
+    // ~53 weeks of activity cells (7 days each)
+    const dayCount = 53 * 7;
+
+    it("uses up-right for days in the last 10 weeks (right edge)", () => {
+      // last day of the calendar (today / rightmost column)
+      expect(getActivityBalloonPos(dayCount - 1, dayCount)).toBe("up-right");
+      // first day of the 10th week from the end
+      expect(getActivityBalloonPos((53 - 10) * 7, dayCount)).toBe("up-right");
+    });
+
+    it("uses up-left for days in the first 10 weeks (left edge)", () => {
+      expect(getActivityBalloonPos(0, dayCount)).toBe("up-left");
+      expect(getActivityBalloonPos(9 * 7 + 6, dayCount)).toBe("up-left");
+    });
+
+    it("uses centered up for days in the middle", () => {
+      expect(getActivityBalloonPos(26 * 7, dayCount)).toBe("up");
+      expect(getActivityBalloonPos(10 * 7, dayCount)).toBe("up");
+      expect(getActivityBalloonPos((53 - 11) * 7 + 6, dayCount)).toBe("up");
+    });
+  });
+});
diff --git a/frontend/src/ts/elements/test-activity.ts b/frontend/src/ts/elements/test-activity.ts
index 7d2bae4b95de..e8170af59572 100644
--- a/frontend/src/ts/elements/test-activity.ts
+++ b/frontend/src/ts/elements/test-activity.ts
@@ -3,6 +3,27 @@ import {
   TestActivityMonth,
 } from "./test-activity-calendar";
 
+// Non-centered balloons on edge weeks so tooltips aren't clipped by viewport.
+const BALLOON_EDGE_WEEKS = 10;
+
+export type ActivityBalloonPos = "up" | "up-left" | "up-right";
+
+/** Right-edge cells → `up-right` (extends left); left-edge → `up-left`. */
+export function getActivityBalloonPos(
+  dayIndex: number,
+  dayCount: number,
+): ActivityBalloonPos {
+  const weekCount = Math.ceil(dayCount / 7);
+  const week = Math.floor(dayIndex / 7);
+  if (week >= weekCount - BALLOON_EDGE_WEEKS) {
+    return "up-right";
+  }
+  if (week < BALLOON_EDGE_WEEKS) {
+    return "up-left";
+  }
+  return "up";
+}
+
 export function init(
   element: HTMLElement,
   calendar?: TestActivityCalendar,
@@ -51,12 +72,18 @@ export function update(
     }
   }
 
-  for (const day of calendar.getDays()) {
+  const days = calendar.getDays();
+  for (let i = 0; i < days.length; i++) {
+    const day = days[i];
+    if (day === undefined) continue;
     const elem = document.createElement("div");
     elem.setAttribute("data-level", day.level);
     if (day.label !== undefined) {
       elem.setAttribute("aria-label", day.label);
-      elem.setAttribute("data-balloon-pos", "up");
+      elem.setAttribute(
+        "data-balloon-pos",
+        getActivityBalloonPos(i, days.length),
+      );
     }
     container.appendChild(elem);
   }