Fix clipping of graphics effects in high dpi mode

Consider this pseudo code:
setSystemClip(region);
setSystemTransform( scale 2x );
setSystemTransform( scale 2x );

The second call to setSystemTransform should be a noop. Yet, with
the current code in setSystemTransform, the region would be scaled
twice by 2x and thus the clipping would be incorrect.

Fix that by saving the original untransformed clip and in
setSystemTransform recalculate the effective transform.

This is also a better fix for QTBUG-44067 /
sha: 083a945c166b325298a43ba591b1338d1b0f99b6, since with this patch
the order in which system clip, viewport and transform are set does
no longer matter.

Task-number: QTBUG-57257
Task-number: QTBUG-55698
Change-Id: Ibc232822e97ab116f7173a0cc50bba5a367619d8
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/gui/painting/qpaintengine.cpp

@@ -929,11 +929,11 @@ QPoint QPaintEngine::coordinateOffset() const
 void QPaintEngine::setSystemClip(const QRegion &region)
 {
     Q_D(QPaintEngine);
-    d->systemClip = region;
+    d->baseSystemClip = region;
     // Be backward compatible and only call d->systemStateChanged()
     // if we currently have a system transform/viewport set.
+    d->updateSystemClip();
     if (d->hasSystemTransform || d->hasSystemViewport) {
-        d->transformSystemClip();
         d->systemStateChanged();
     }
 }

src/gui/painting/qpaintengine_p.h

@@ -71,6 +71,7 @@ public:
 
     QPaintDevice *pdev;
     QPaintEngine *q_ptr;
+    QRegion baseSystemClip;
     QRegion systemClip;
     QRect systemRect;
     QRegion systemViewport;
@@ -79,8 +80,9 @@ public:
     uint hasSystemTransform : 1;
     uint hasSystemViewport : 1;
 
-    inline void transformSystemClip()
+    inline void updateSystemClip()
     {
+        systemClip = baseSystemClip;
         if (systemClip.isEmpty())
             return;
 
@@ -104,15 +106,30 @@ public:
     inline void setSystemTransform(const QTransform &xform)
     {
         systemTransform = xform;
-        if ((hasSystemTransform = !xform.isIdentity()) || hasSystemViewport)
+        hasSystemTransform = !xform.isIdentity();
-            transformSystemClip();
+        updateSystemClip();
-        systemStateChanged();
+        if (q_ptr->state)
+            systemStateChanged();
     }
 
     inline void setSystemViewport(const QRegion &region)
     {
         systemViewport = region;
         hasSystemViewport = !systemViewport.isEmpty();
+        updateSystemClip();
+        if (q_ptr->state)
+            systemStateChanged();
+    }
+
+    inline void setSystemTransformAndViewport(const QTransform &xform, const QRegion &region)
+    {
+        systemTransform = xform;
+        hasSystemTransform = !xform.isIdentity();
+        systemViewport = region;
+        hasSystemViewport = !systemViewport.isEmpty();
+        updateSystemClip();
+        if (q_ptr->state)
+            systemStateChanged();
     }
 
     virtual void systemStateChanged() { }

src/widgets/kernel/qwidget.cpp

@@ -2102,14 +2102,15 @@ QRegion QWidgetPrivate::clipRegion() const
     return r;
 }
 
-void QWidgetPrivate::setSystemClip(QPaintDevice *paintDevice, const QRegion &region)
+void QWidgetPrivate::setSystemClip(QPaintEngine *paintEngine, qreal devicePixelRatio, const QRegion &region)
 {
 // Transform the system clip region from device-independent pixels to device pixels
-    QPaintEngine *paintEngine = paintDevice->paintEngine();
     QTransform scaleTransform;
-    const qreal devicePixelRatio = paintDevice->devicePixelRatioF();
     scaleTransform.scale(devicePixelRatio, devicePixelRatio);
-    paintEngine->d_func()->systemClip = scaleTransform.map(region);
+
+    paintEngine->d_func()->baseSystemClip = region;
+    paintEngine->d_func()->setSystemTransform(scaleTransform);
+
 }
 
 #ifndef QT_NO_GRAPHICSEFFECT
@@ -5189,6 +5190,7 @@ void QWidget::render(QPainter *painter, const QPoint &targetOffset,
     // Save current system clip, viewport and transform,
     const QTransform oldTransform = enginePriv->systemTransform;
     const QRegion oldSystemClip = enginePriv->systemClip;
+    const QRegion oldBaseClip = enginePriv->baseSystemClip;
     const QRegion oldSystemViewport = enginePriv->systemViewport;
 
     // This ensures that all painting triggered by render() is clipped to the current engine clip.
@@ -5202,9 +5204,8 @@ void QWidget::render(QPainter *painter, const QPoint &targetOffset,
     d->render(target, targetOffset, toBePainted, renderFlags);
 
     // Restore system clip, viewport and transform.
-    enginePriv->setSystemViewport(oldSystemViewport);
+    enginePriv->baseSystemClip = oldBaseClip;
-    enginePriv->setSystemTransform(oldTransform);
+    enginePriv->setSystemTransformAndViewport(oldTransform, oldSystemViewport);
-    enginePriv->systemClip = oldSystemClip;
 
     // Restore shared painter.
     d->setSharedPainter(oldPainter);
@@ -5495,12 +5496,12 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
             QWidgetPaintContext context(pdev, rgn, offset, flags, sharedPainter, backingStore);
             sourced->context = &context;
             if (!sharedPainter) {
-                setSystemClip(pdev, rgn.translated(offset));
+                setSystemClip(pdev->paintEngine(), pdev->devicePixelRatioF(), rgn.translated(offset));
                 QPainter p(pdev);
                 p.translate(offset);
                 context.painter = &p;
                 graphicsEffect->draw(&p);
-                setSystemClip(pdev, QRegion());
+                setSystemClip(pdev->paintEngine(), 1, QRegion());
             } else {
                 context.painter = sharedPainter;
                 if (sharedPainter->worldTransform() != sourced->lastEffectTransform) {
@@ -5509,7 +5510,9 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                 }
                 sharedPainter->save();
                 sharedPainter->translate(offset);
+                setSystemClip(sharedPainter->paintEngine(), sharedPainter->device()->devicePixelRatioF(), rgn.translated(offset));
                 graphicsEffect->draw(sharedPainter);
+                setSystemClip(sharedPainter->paintEngine(), 1, QRegion());
                 sharedPainter->restore();
             }
             sourced->context = 0;
@@ -5561,7 +5564,7 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
 
 #endif
                 if (sharedPainter)
-                    setSystemClip(pdev, toBePainted);
+                    setSystemClip(pdev->paintEngine(), pdev->devicePixelRatioF(), toBePainted);
                 else
                     paintEngine->d_func()->systemRect = q->data->crect;
 
@@ -5579,7 +5582,7 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                 }
 
                 if (!sharedPainter)
-                    setSystemClip(pdev, toBePainted.translated(offset));
+                    setSystemClip(pdev->paintEngine(), pdev->devicePixelRatioF(), toBePainted.translated(offset));
 
                 if (!onScreen && !asRoot && !isOpaque && q->testAttribute(Qt::WA_TintedBackground)) {
 #ifndef QT_NO_OPENGL
@@ -5647,7 +5650,7 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                 else
                     paintEngine->d_func()->currentClipDevice = 0;
 
-                setSystemClip(pdev, QRegion());
+                setSystemClip(pdev->paintEngine(), 1, QRegion());
             }
             q->setAttribute(Qt::WA_WState_InPaintEvent, false);
             if (Q_UNLIKELY(q->paintingActive()))

src/widgets/kernel/qwidget_p.h

@@ -409,7 +409,7 @@ public:
 
     QRect clipRect() const;
     QRegion clipRegion() const;
-    void setSystemClip(QPaintDevice *paintDevice, const QRegion &region);
+    void setSystemClip(QPaintEngine *paintEngine, qreal devicePixelRatio, const QRegion &region);
     void subtractOpaqueChildren(QRegion &rgn, const QRect &clipRect) const;
     void subtractOpaqueSiblings(QRegion &source, bool *hasDirtySiblingsAbove = 0,
                                 bool alsoNonOpaque = false) const;