Render in place: a track's output to a new sibling, source to the bench

tests/test_view_mode_model.cpp

@@ -1739,6 +1739,48 @@ static void testLaneMintingEmptyFolderNotSplit() {
 
 // -- D2.6 JSON round-trip with lane index + membership -----------------------
 
+// An EXPLICIT Arrange record is new: the shipped "tag selected tracks -> Arrange"
+// action untags instead, so until now Arrange was only ever represented by absence.
+// The render-in-place verb writes one, because the record — not the behaviour — is
+// what the panel's auto-tag detector defers to. It must be indistinguishable from
+// absence everywhere else.
+static void testExplicitArrangeRecordRoundTripsAndBehavesLikeAbsence() {
+    ViewModeModel vm;
+    vm.membership().tag("{TAGGED-ARRANGE}", kArrangeModeId);
+    // "{UNTAGGED}" is deliberately never tagged — the comparison partner.
+
+    const std::string json = vm.serialize();
+    const auto back = ViewModeModel::deserialize(json);
+    CHECK(back.has_value());
+    CHECK(back && *back == vm);
+    if (back) CHECK(back->serialize() == json);
+
+    // The record survives as a record, not collapsed away on the round-trip.
+    if (back) {
+        const Membership* m = back->membership().query("{TAGGED-ARRANGE}");
+        CHECK(m != nullptr);
+        CHECK(m && m->modeIds == std::set<std::string>{kArrangeModeId});
+        CHECK(back->membership().query("{UNTAGGED}") == nullptr);
+    }
+
+    // Membership answers identically for the record and for its absence, in BOTH
+    // modes — that equivalence is what makes writing the record free of behaviour.
+    const auto checkEquivalent = [](const ViewModeModel& m) {
+        CHECK(m.leafBelongsToMode("{TAGGED-ARRANGE}", kArrangeModeId) ==
+              m.leafBelongsToMode("{UNTAGGED}", kArrangeModeId));
+        CHECK(m.leafBelongsToMode("{TAGGED-ARRANGE}", kArrangeModeId));
+        CHECK(m.leafBelongsToMode("{TAGGED-ARRANGE}", kDesignModeId) ==
+              m.leafBelongsToMode("{UNTAGGED}", kDesignModeId));
+        CHECK(!m.leafBelongsToMode("{TAGGED-ARRANGE}", kDesignModeId));
+    };
+    checkEquivalent(vm);
+    if (back) checkEquivalent(*back);  // and after a save/reload round-trip
+
+    // untag() still returns it to absence, so the existing way out still works.
+    CHECK(vm.membership().untag("{TAGGED-ARRANGE}"));
+    CHECK(vm.membership().query("{TAGGED-ARRANGE}") == nullptr);
+}
+
 static void testLaneJsonRoundTrip() {
     ViewModeModel vm;
     CHECK(vm.modes().add(Mode{"mixdown", "Mixdown", 2}));
@@ -1922,6 +1964,7 @@ int main() {
     testLaneMintingShowBothNotForceSplit();
     testLaneMintingSingleModeLeafVisibleOnceNoSplit();
     testLaneMintingEmptyFolderNotSplit();
+    testExplicitArrangeRecordRoundTripsAndBehavesLikeAbsence();
     testLaneJsonRoundTrip();
     testLaneMalformedJson();