diff --git a/src/gui/kernel/qpalette.cpp b/src/gui/kernel/qpalette.cpp
index 33a885b510d..081af380745 100644
--- a/src/gui/kernel/qpalette.cpp
+++ b/src/gui/kernel/qpalette.cpp
@@ -35,14 +35,25 @@ class QPalettePrivate
 public:
     class Data : public QSharedData {
     public:
-        Data() : ser_no(qt_palette_count++) { }
+        // Every instance of Data has to have a unique serial number, even
+        // if it gets created by copying another - we wouldn't create a copy
+        // in the first place if the serial number should be the same!
+        Data(const Data &other)
+            : QSharedData(other)
+        {
+            for (int grp = 0; grp < int(QPalette::NColorGroups); grp++) {
+                for (int role = 0; role < int(QPalette::NColorRoles); role++)
+                    br[grp][role] = other.br[grp][role];
+            }
+        }
+        Data() = default;
 
         QBrush br[QPalette::NColorGroups][QPalette::NColorRoles];
-        const int ser_no;
+        const int ser_no = qt_palette_count++;
     };
 
     QPalettePrivate(const QExplicitlySharedDataPointer<Data> &data)
-        : ref(1), detach_no(0), data(data)
+        : ref(1), data(data)
     { }
     QPalettePrivate()
         : QPalettePrivate(QExplicitlySharedDataPointer<Data>(new Data))
@@ -50,7 +61,8 @@ public:
 
     QAtomicInt ref;
     QPalette::ResolveMask resolveMask = {0};
-    int detach_no;
+    static inline int qt_palette_private_count = 0;
+    const int detach_no = ++qt_palette_private_count;
     QExplicitlySharedDataPointer<Data> data;
 };
 
@@ -853,7 +865,6 @@ void QPalette::detach()
             delete d;
         d = x;
     }
-    ++d->detach_no;
 }
 
 /*!
diff --git a/tests/auto/gui/kernel/qpalette/tst_qpalette.cpp b/tests/auto/gui/kernel/qpalette/tst_qpalette.cpp
index 150d58e6f4f..2e9a484b8b2 100644
--- a/tests/auto/gui/kernel/qpalette/tst_qpalette.cpp
+++ b/tests/auto/gui/kernel/qpalette/tst_qpalette.cpp
@@ -22,6 +22,7 @@ private Q_SLOTS:
     void noBrushesSetForDefaultPalette();
     void cannotCheckIfInvalidBrushSet();
     void checkIfBrushForCurrentGroupSet();
+    void cacheKey();
 };
 
 void tst_QPalette::roleValues_data()
@@ -264,5 +265,61 @@ void tst_QPalette::checkIfBrushForCurrentGroupSet()
     QVERIFY(p.isBrushSet(QPalette::Current, QPalette::Link));
 }
 
+void tst_QPalette::cacheKey()
+{
+    const QPalette defaultPalette;
+    // precondition: all palettes are expected to have contrasting text on base
+    QVERIFY(defaultPalette.base() != defaultPalette.text());
+    const auto defaultCacheKey = defaultPalette.cacheKey();
+    const auto defaultSerNo = defaultCacheKey >> 32;
+    const auto defaultDetachNo = defaultCacheKey & 0xffffffff;
+
+    QPalette copyDifferentData(defaultPalette);
+    QPalette copyDifferentMask(defaultPalette);
+    QPalette copyDifferentMaskAndData(defaultPalette);
+
+    QCOMPARE(defaultPalette.cacheKey(), copyDifferentData.cacheKey());
+
+    // deep detach of both private and data
+    copyDifferentData.setBrush(QPalette::Base, defaultPalette.text());
+    const auto differentDataKey = copyDifferentData.cacheKey();
+    const auto differentDataSerNo = differentDataKey >> 32;
+    const auto differentDataDetachNo = differentDataKey & 0xffffffff;
+
+    QCOMPARE_NE(copyDifferentData.cacheKey(), defaultCacheKey);
+    QCOMPARE(defaultPalette.cacheKey(), defaultCacheKey);
+
+    // shallow detach, both privates reference the same data
+    copyDifferentMask.setResolveMask(0xffffffffffffffff);
+    const auto differentMaskKey = copyDifferentMask.cacheKey();
+    const auto differentMaskSerNo = differentMaskKey >> 32;
+    const auto differentMaskDetachNo = differentMaskKey & 0xffffffff;
+    QCOMPARE(differentMaskSerNo, defaultSerNo);
+    QCOMPARE_NE(differentMaskSerNo, defaultDetachNo);
+    QCOMPARE_NE(differentMaskKey, defaultCacheKey);
+    QCOMPARE_NE(differentMaskKey, differentDataKey);
+
+    // shallow detach, both privates reference the same data
+    copyDifferentMaskAndData.setResolveMask(0xeeeeeeeeeeeeeeee);
+    const auto modifiedCacheKey = copyDifferentMaskAndData.cacheKey();
+    QCOMPARE_NE(modifiedCacheKey, copyDifferentMask.cacheKey());
+    QCOMPARE_NE(modifiedCacheKey, defaultCacheKey);
+    QCOMPARE_NE(modifiedCacheKey, copyDifferentData.cacheKey());
+    QCOMPARE_NE(copyDifferentMask.cacheKey(), defaultCacheKey);
+
+    // full detach - both key elements are different
+    copyDifferentMaskAndData.setBrush(QPalette::Base, defaultPalette.text());
+    const auto modifiedAllKey = copyDifferentMaskAndData.cacheKey();
+    const auto modifiedAllSerNo = modifiedAllKey >> 32;
+    const auto modifiedAllDetachNo = modifiedAllKey & 0xffffffff;
+    QCOMPARE_NE(modifiedAllSerNo, defaultSerNo);
+    QCOMPARE_NE(modifiedAllDetachNo, defaultDetachNo);
+
+    QCOMPARE_NE(modifiedAllKey, copyDifferentMask.cacheKey());
+    QCOMPARE_NE(modifiedAllKey, defaultCacheKey);
+    QCOMPARE_NE(modifiedAllKey, differentDataKey);
+    QCOMPARE_NE(modifiedAllKey, modifiedCacheKey);
+}
+
 QTEST_MAIN(tst_QPalette)
 #include "tst_qpalette.moc"