https://bugreports.qt.io/browse/QTBUG-130767
https://bugs.kde.org/show_bug.cgi?id=494804
https://codereview.qt-project.org/c/qt/qtdeclarative/+/604180
(+required https://code.qt.io/cgit/qt/qtdeclarative.git/commit/?id=104b0d6e88)
--- a/src/qml/jsruntime/qv4qobjectwrapper_p.h
+++ b/src/qml/jsruntime/qv4qobjectwrapper_p.h
@@ -224,2 +224,15 @@
 
+// We generally musn't pass ReturnedValue as arguments to other functions.
+// In this case, we do it solely for marking purposes so it's fine.
+inline void markIfPastMarkWeakValues(ExecutionEngine *engine, ReturnedValue rv)
+{
+    const auto gcState = engine->memoryManager->gcStateMachine->state;
+    if (gcState != GCStateMachine::Invalid && gcState >= GCState::MarkWeakValues) {
+        QV4::WriteBarrier::markCustom(engine, [rv](QV4::MarkStack *ms) {
+            auto *m = StaticValue::fromReturnedValue(rv).m();
+            m->mark(ms);
+        });
+    }
+}
+
 inline ReturnedValue QObjectWrapper::wrap(ExecutionEngine *engine, QObject *object)
@@ -235,3 +248,5 @@
 
-    return wrap_slowPath(engine, object);
+    const auto rv = wrap_slowPath(engine, object);
+    markIfPastMarkWeakValues(engine, rv);
+    return rv;
 }
@@ -244,3 +259,5 @@
 
-    return wrapConst_slowPath(engine, object);
+    const auto rv = wrapConst_slowPath(engine, object);
+    markIfPastMarkWeakValues(engine, rv);
+    return rv;
 }
--- a/src/qml/memory/qv4mm.cpp
+++ b/src/qml/memory/qv4mm.cpp
@@ -62,2 +62,4 @@
 Q_DECLARE_LOGGING_CATEGORY(lcGcAllocatorStats)
+Q_LOGGING_CATEGORY(lcGcStateTransitions, "qt.qml.gc.stateTransitions")
+Q_DECLARE_LOGGING_CATEGORY(lcGcStateTransitions)
 
@@ -682,3 +684,3 @@
     that->mm->engine->isGCOngoing = true;
-    return MarkGlobalObject;
+    return GCState::MarkGlobalObject;
 }
@@ -688,3 +690,3 @@
     that->mm->engine->markObjects(that->mm->m_markStack.get());
-    return MarkJSStack;
+    return GCState::MarkJSStack;
 }
@@ -694,3 +696,3 @@
     that->mm->collectFromJSStack(that->mm->markStack());
-    return InitMarkPersistentValues;
+    return GCState::InitMarkPersistentValues;
 }
@@ -700,5 +702,5 @@
     if (!that->mm->m_persistentValues)
-        return InitMarkWeakValues; // no persistent values to mark
+        return GCState::InitMarkWeakValues; // no persistent values to mark
     stateData = GCIteratorStorage { that->mm->m_persistentValues->begin() };
-    return MarkPersistentValues;
+    return GCState::MarkPersistentValues;
 }
@@ -719,3 +721,3 @@
     if (wasDrainNecessary(markStack, that->deadline) && that->deadline.hasExpired())
-        return MarkPersistentValues;
+        return GCState::MarkPersistentValues;
     PersistentValueStorage::Iterator& it = get<GCIteratorStorage>(stateData).it;
@@ -724,3 +726,3 @@
         if (!it.p)
-            return InitMarkWeakValues;
+            return GCState::InitMarkWeakValues;
         if (Managed *m = (*it).as<Managed>())
@@ -729,3 +731,3 @@
     }
-    return MarkPersistentValues;
+    return GCState::MarkPersistentValues;
 }
@@ -735,3 +737,3 @@
     stateData = GCIteratorStorage { that->mm->m_weakValues->begin() };
-    return MarkWeakValues;
+    return GCState::MarkWeakValues;
 }
@@ -742,3 +744,3 @@
     if (wasDrainNecessary(markStack, that->deadline) && that->deadline.hasExpired())
-        return MarkWeakValues;
+        return GCState::MarkWeakValues;
     PersistentValueStorage::Iterator& it = get<GCIteratorStorage>(stateData).it;
@@ -747,3 +749,3 @@
         if (!it.p)
-            return MarkDrain;
+            return GCState::MarkDrain;
         QObjectWrapper *qobjectWrapper = (*it).as<QObjectWrapper>();
@@ -768,3 +770,3 @@
     }
-    return MarkWeakValues;
+    return GCState::MarkWeakValues;
 }
@@ -775,3 +777,3 @@
         that->mm->markStack()->drain();
-        return MarkReady;
+        return GCState::MarkReady;
     }
@@ -779,4 +781,4 @@
     return drainState == MarkStack::DrainState::Complete
-            ? MarkReady
-            : MarkDrain;
+            ? GCState::MarkReady
+            : GCState::MarkDrain;
 }
@@ -786,3 +788,3 @@
     //Possibility to do some clean up, stat printing, etc...
-    return InitCallDestroyObjects;
+    return GCState::InitCallDestroyObjects;
 }
@@ -803,5 +805,5 @@
     if (!that->mm->m_weakValues)
-        return FreeWeakMaps; // no need to call destroy objects
+        return GCState::FreeWeakMaps; // no need to call destroy objects
     stateData = GCIteratorStorage { that->mm->m_weakValues->begin() };
-    return CallDestroyObjects;
+    return GCState::CallDestroyObjects;
 }
@@ -818,3 +820,3 @@
         if (!it.p)
-            return FreeWeakMaps;
+            return GCState::FreeWeakMaps;
         Managed *m = (*it).managed();
@@ -828,3 +830,3 @@
     }
-    return CallDestroyObjects;
+    return GCState::CallDestroyObjects;
 }
@@ -845,3 +847,3 @@
     freeWeakMaps(that->mm);
-    return FreeWeakSets;
+    return GCState::FreeWeakSets;
 }
@@ -863,3 +865,3 @@
     freeWeakSets(that->mm);
-    return HandleQObjectWrappers;
+    return GCState::HandleQObjectWrappers;
 }
@@ -869,3 +871,3 @@
     that->mm->cleanupDeletedQObjectWrappersInSweep();
-    return DoSweep;
+    return GCState::DoSweep;
 }
@@ -893,3 +895,3 @@
 
-    return Invalid;
+    return GCState::Invalid;
 }
@@ -1493,4 +1495,8 @@
             }
+            qCDebug(lcGcStateTransitions) << "Preparing to execute the"
+                                          << QMetaEnum::fromType<GCState>().key(state) << "state";
             GCStateInfo& stateInfo = stateInfoMap[int(state)];
             state = stateInfo.execute(this, stateData);
+            qCDebug(lcGcStateTransitions) << "Transitioning to the"
+                                          << QMetaEnum::fromType<GCState>().key(state) << "state";
             if (stateInfo.breakAfter)
@@ -1507,4 +1513,8 @@
         while (state != GCState::Invalid) {
+            qCDebug(lcGcStateTransitions) << "Preparing to execute the"
+                                          << QMetaEnum::fromType<GCState>().key(state) << "state";
             GCStateInfo& stateInfo = stateInfoMap[int(state)];
             state = stateInfo.execute(this, stateData);
+            qCDebug(lcGcStateTransitions) << "Transitioning to the"
+                                          << QMetaEnum::fromType<GCState>().key(state) << "state";
         }
@@ -1516 +1526,3 @@
 QT_END_NAMESPACE
+
+#include "moc_qv4mm_p.cpp"
--- a/src/qml/memory/qv4mm_p.h
+++ b/src/qml/memory/qv4mm_p.h
@@ -30,22 +30,2 @@
 
-enum GCState {
-    MarkStart = 0,
-    MarkGlobalObject,
-    MarkJSStack,
-    InitMarkPersistentValues,
-    MarkPersistentValues,
-    InitMarkWeakValues,
-    MarkWeakValues,
-    MarkDrain,
-    MarkReady,
-    InitCallDestroyObjects,
-    CallDestroyObjects,
-    FreeWeakMaps,
-    FreeWeakSets,
-    HandleQObjectWrappers,
-    DoSweep,
-    Invalid,
-    Count,
-};
-
 struct GCData { virtual ~GCData(){};};
@@ -55,11 +35,39 @@
 };
-struct GCStateMachine;
-
-struct GCStateInfo {
-    using ExtraData = std::variant<std::monostate, GCIteratorStorage>;
-    GCState (*execute)(GCStateMachine *, ExtraData &) = nullptr;  // Function to execute for this state, returns true if ready to transition
-    bool breakAfter{false};
-};
 
 struct GCStateMachine {
+    Q_GADGET_EXPORT(Q_QML_EXPORT)
+
+public:
+    enum GCState {
+        MarkStart = 0,
+        MarkGlobalObject,
+        MarkJSStack,
+        InitMarkPersistentValues,
+        MarkPersistentValues,
+        InitMarkWeakValues,
+        MarkWeakValues,
+        MarkDrain,
+        MarkReady,
+        InitCallDestroyObjects,
+        CallDestroyObjects,
+        FreeWeakMaps,
+        FreeWeakSets,
+        HandleQObjectWrappers,
+        DoSweep,
+        Invalid,
+        Count,
+    };
+    Q_ENUM(GCState)
+
+    struct StepTiming {
+        qint64 rolling_sum = 0;
+        qint64 count = 0;
+    };
+
+    struct GCStateInfo {
+        using ExtraData = std::variant<std::monostate, GCIteratorStorage>;
+        GCState (*execute)(GCStateMachine *, ExtraData &) = nullptr;  // Function to execute for this state, returns true if ready to transition
+        bool breakAfter{false};
+    };
+
     using ExtraData = GCStateInfo::ExtraData;
@@ -96,2 +104,4 @@
 
+using GCState = GCStateMachine::GCState;
+using GCStateInfo = GCStateMachine::GCStateInfo;