fix(studio): hide a group's lane toggle when it automates nothing

A track header already gates its own automation toggle on having
something to disclose (`disclosable`). A group's did not, so every group
offered a wave button that opened an empty row — which is how the author
learns the group has no automation, one click too late.

Gated on the same count the badge already used. Nothing becomes
unreachable: automation appears on a group by being written, from the
rack or a keyframe, not by opening this.

The suite had nothing covering the toggle's presence, so the test is new
and was mutation-checked — forcing the condition true fails it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/studio/src/player/components/TimelineGroupHeader.tsx

@@ -141,27 +141,33 @@ export function TimelineGroupHeader({
           auditionSpans={auditionSpans}
           onOpenRack={onOpenFxRack}
         />
-        <button
-          type="button"
-          tabIndex={-1}
-          aria-expanded={isLaneOpen}
-          aria-label={`${isLaneOpen ? "Hide" : "Show"} ${label} lanes`}
-          title={`${isLaneOpen ? "Hide" : "Show"} lanes`}
-          // Anchored right, matching every other header's lane toggle.
-          className={`ml-auto flex h-6 items-center justify-center gap-0.5 rounded border-0 bg-transparent px-1 text-[11px] leading-none focus-visible:outline focus-visible:outline-1 focus-visible:outline-[#3CE6AC] ${
-            isLaneOpen ? "text-[#3CE6AC]" : "text-white/55 hover:text-white"
-          }`}
-          onPointerDown={(event) => event.stopPropagation()}
-          onClick={(event) => {
-            event.stopPropagation();
-            onToggleLanes();
-          }}
-        >
-          <span aria-hidden="true">∿</span>
-          {laneCount > 0 && (
+        {/* No lanes, no control: an author who opens it meets an empty row and
+            learns nothing. A track header already gates its own `∿` this way
+            (`disclosable`); the group's was the one that still offered a
+            disclosure over nothing. Automation appears by being written — from
+            the rack or a keyframe — not by opening this, so nothing is
+            unreachable while it is hidden. */}
+        {laneCount > 0 && (
+          <button
+            type="button"
+            tabIndex={-1}
+            aria-expanded={isLaneOpen}
+            aria-label={`${isLaneOpen ? "Hide" : "Show"} ${label} lanes`}
+            title={`${isLaneOpen ? "Hide" : "Show"} lanes`}
+            // Anchored right, matching every other header's lane toggle.
+            className={`ml-auto flex h-6 items-center justify-center gap-0.5 rounded border-0 bg-transparent px-1 text-[11px] leading-none focus-visible:outline focus-visible:outline-1 focus-visible:outline-[#3CE6AC] ${
+              isLaneOpen ? "text-[#3CE6AC]" : "text-white/55 hover:text-white"
+            }`}
+            onPointerDown={(event) => event.stopPropagation()}
+            onClick={(event) => {
+              event.stopPropagation();
+              onToggleLanes();
+            }}
+          >
+            <span aria-hidden="true">∿</span>
             <span className="text-[9px] tabular-nums text-white/55">{laneCount}</span>
-          )}
-        </button>
+          </button>
+        )}
       </div>
     </div>
   );

packages/studio/src/player/components/TimelineGroupRow.test.tsx

@@ -36,7 +36,7 @@ const GROUP: TimelineTrackGroupInfo = {
   hidden: false,
 };
 
-function renderRow() {
+function renderRow(overrides: Partial<TimelineTrackGroupInfo> = {}) {
   const onSetAudioGroupAttributeQuiet = vi.fn();
   const onSetElementAttributeQuiet = vi.fn();
   const host = document.createElement("div");
@@ -47,7 +47,7 @@ function renderRow() {
         <TimelineGroupRow
           index={0}
           rowKey={0}
-          group={GROUP}
+          group={{ ...GROUP, ...overrides }}
           logicalRow={{ id: "g", level: 1, kind: "track" } as never}
           top={0}
           height={48}
@@ -93,4 +93,28 @@ describe("TimelineGroupRow", () => {
     // The members are the point: not one write reaches them.
     expect(onSetElementAttributeQuiet).not.toHaveBeenCalled();
   });
+
+  // A disclosure over nothing tells the author their group has no automation
+  // only AFTER they open an empty row. Track headers already gate their own
+  // toggle on having something to disclose; the group's did not.
+  it("hides the lane toggle until the group actually automates something", () => {
+    const laneToggle = (host: HTMLElement) =>
+      Array.from(host.querySelectorAll("button")).find((b) =>
+        /lanes$/.test(b.getAttribute("aria-label") ?? ""),
+      );
+
+    expect(laneToggle(renderRow().host)).toBeUndefined();
+
+    const automated = renderRow({
+      fxChain: JSON.stringify({
+        version: 1,
+        nodes: [{ type: "peaking", id: "p1", params: { frequency: 1000, gain: -3, q: 1 } }],
+      }),
+      automation: JSON.stringify({
+        version: 1,
+        lanes: [{ target: "fx.p1.gain", points: [{ t: 0, v: 0 }] }],
+      }),
+    });
+    expect(laneToggle(automated.host)).toBeDefined();
+  });
 });