diff --git a/frontend/src/features/tickets/TicketViewportSelect.test.tsx b/frontend/src/features/tickets/TicketViewportSelect.test.tsx
index 0b07c2a..8961124 100644
--- a/frontend/src/features/tickets/TicketViewportSelect.test.tsx
+++ b/frontend/src/features/tickets/TicketViewportSelect.test.tsx
@@ -40,7 +40,8 @@ describe("TicketViewportSelect", () => {
       name: "filter by assignee",
     });
     expect(listbox.parentElement).toBe(document.body);
-    expect(listbox.style.zIndex).toBe(String(zIndex.menuDropdown));
+    expect(listbox.style.zIndex).toBe(String(zIndex.transientPopup));
+    expect(zIndex.transientPopup).toBeGreaterThan(zIndex.floatingWindow);
 
     fireEvent.click(screen.getByRole("option", { name: "Alpha" }));
     expect(onChange).toHaveBeenCalledWith("alpha");
@@ -102,4 +103,3 @@ describe("TicketViewportSelect", () => {
     expect(screen.queryByRole("listbox")).toBeNull();
   });
 });
-
diff --git a/frontend/src/features/tickets/TicketViewportSelect.tsx b/frontend/src/features/tickets/TicketViewportSelect.tsx
index b67b0a9..66ce9eb 100644
--- a/frontend/src/features/tickets/TicketViewportSelect.tsx
+++ b/frontend/src/features/tickets/TicketViewportSelect.tsx
@@ -158,7 +158,7 @@ export function TicketViewportSelect({
               left: position.left,
               width: position.width,
               maxHeight: position.maxHeight,
-              zIndex: zIndex.menuDropdown,
+              zIndex: zIndex.transientPopup,
             }}
           >
             {options.map((option) => (
diff --git a/frontend/src/shared/ui/zIndex.ts b/frontend/src/shared/ui/zIndex.ts
index 00cacd9..7e2035f 100644
--- a/frontend/src/shared/ui/zIndex.ts
+++ b/frontend/src/shared/ui/zIndex.ts
@@ -7,9 +7,10 @@
  * LayoutGrid/LeafView (G5).
  *
  * Consumers: `FloatingWindow` (floatingWindow), `MenuBar` dropdowns
- * (menuDropdown), `TicketPicker` (floatingWindowNested), background-task toasts
- * (toast). Introduced by lot B (#18) and adopted as the canonical scale by lot A
- * (#16). See the sprint scoping note `ui-rework-sprint-scoping-contracts`.
+ * (menuDropdown), `TicketPicker` (floatingWindowNested), transient popups opened
+ * from floating windows (transientPopup), background-task toasts (toast).
+ * Introduced by lot B (#18) and adopted as the canonical scale by lot A (#16).
+ * See the sprint scoping note `ui-rework-sprint-scoping-contracts`.
  */
 export const zIndex = {
   /** Dropdown menus / popovers anchored to a control. */
@@ -19,6 +20,8 @@ export const zIndex = {
   /** A floating window opened from within another floating window (e.g. a
    *  ticket picker launched from a sprint/link dialog). Sits above it. */
   floatingWindowNested: 60,
+  /** Viewport-positioned popups that must float above their owning window. */
+  transientPopup: 65,
   /** Transient toasts, always on top. */
   toast: 70,
 } as const;