fix(ui): a late form takes a dialog's focus once, even if it arrived elsewhere

The watch stayed armed when the form arrived while the focus was off the close button, so a
later change in the body pulled the focus off a close button the user went back to. It now
disarms on arrival and re-arms only when the focus is taken away.

frontend/src/components/ui/Modal.purpose.dom.test.tsx

@@ -116,6 +116,28 @@ describe("Modal initial focus by purpose (C4)", () => {
     expect(document.activeElement).toBe(cancel);
   });
 
+  // …and it stays the user's: the form disarms the watch when it arrives, so a later change in the
+  // body (a hint appearing as they type) cannot pull the focus off a ✕ they went back to.
+  it("field that loads late: once arrived, it never takes the focus off the ✕ again", async () => {
+    const view = (ready: boolean, hint = false) => (
+      <Modal initialFocus="field" title="t" onClose={() => {}}>
+        <Late ready={ready} />
+        {hint && <p>hint</p>}
+        <ModalFooter>
+          <button>cancel</button>
+        </ModalFooter>
+      </Modal>
+    );
+    const { rerender } = render(view(false));
+    act(() => screen.getByRole("button", { name: "cancel" }).focus());
+    rerender(view(true));
+    await act(async () => {});
+    act(() => closeBtn().focus());
+    rerender(view(true, true));
+    await act(async () => {});
+    expect(document.activeElement).toBe(closeBtn());
+  });
+
   it("close: the ✕, even with a field in the body", () => {
     render(
       <Modal initialFocus="close" title="t" onClose={() => {}}>

frontend/src/components/ui/Modal.tsx

@@ -106,9 +106,12 @@ export default function Modal({
   // A form that loads after the dialog opens (the issue composer shows a spinner first) has no field
   // to take the focus at open, so it lands on the ✕. When the first control arrives the focus moves
   // there — but only while it still rests on the ✕: a user who has moved it keeps it where it is.
-  // The watch re-arms when the focused control is taken away (a failed load's Retry giving way to
+  // The form takes it ONCE: arrived while the focus was elsewhere, it disarms all the same, or a later
-  // the spinner again): the focus fell to <body>, so it waits on the ✕ for the form (E32 review).
+  // mutation would pull the focus off a ✕ the user went back to (E32 review round 2). The watch
-  // Only <body>: focus in a dialog stacked on top is not this one's to take.
+  // re-arms only when the focus is taken away, not moved: the body has no control again, or the
+  // focused one went (a failed load's Retry giving way to the spinner, or straight to the form) and
+  // the focus fell to <body> — it then waits on the ✕ for the form (E32 review). Only <body>: focus
+  // in a dialog stacked on top is not this one's to take.
   useEffect(() => {
     const body = bodyRef.current;
     if (initialFocus !== "field" || !body || focusablesIn(body).length > 0) return;
@@ -119,9 +122,13 @@ export default function Modal({
         armed = true;
       }
       const first = focusablesIn(body)[0];
-      if (!armed || !first || document.activeElement !== closeRef.current) return;
+      if (!first) {
+        armed = true;
+        return;
+      }
+      if (!armed) return;
       armed = false;
-      first.focus();
+      if (document.activeElement === closeRef.current) first.focus();
     });
     watch.observe(body, { childList: true, subtree: true });
     return () => watch.disconnect();