Add QPlatformIntegration::BackingStoreStaticContents capability

This was a feature in Qt 4, but the only platform that carried it over
to Qt 5 was Windows, in ab76593f18396e693f24066592244ca95e135ea2, and
that's still the situation.

As a first step in possibly implementing this on more platforms, lets
replace the hard-coded check in QWidgetRepaintManager::hasStaticContents
with a proper QPlatformIntegration capability check in the only call site.

Change-Id: I2067109f45116cd8c62facf7224cd748f19e845b
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit cce303b6a70bfc5e7f191529724969ca7509039f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qplatformintegration.cpp

@@ -229,6 +229,11 @@ QPlatformServices *QPlatformIntegration::services() const
     \value ScreenWindowGrabbing The platform supports grabbing window on screen.
     On Wayland, this capability can be reported as \c false. The default implementation
     of hasCapability() returns \c true.
+
+    \value BackingStoreStaticContents The platform backingstore supports static contents.
+    On resize of the backingstore the static contents region is provided, and the backing
+    store is expected to propagate the static content to the resized backing store, without
+    clients needing to repaint the static content region.
  */
 
 /*!

src/gui/kernel/qplatformintegration.h

@@ -99,7 +99,8 @@ public:
         MaximizeUsingFullscreenGeometry,
         PaintEvents,
         RhiBasedRendering,
-        ScreenWindowGrabbing // whether QScreen::grabWindow() is supported
+        ScreenWindowGrabbing, // whether QScreen::grabWindow() is supported
+        BackingStoreStaticContents
     };
 
     virtual ~QPlatformIntegration() { }

src/gui/painting/qbackingstore.cpp

@@ -267,6 +267,13 @@ bool QBackingStore::scroll(const QRegion &area, int dx, int dy)
 */
 void QBackingStore::setStaticContents(const QRegion &region)
 {
+    [[maybe_unused]] static const bool didCheckPlatformSupport = []{
+        const auto *integration = QGuiApplicationPrivate::platformIntegration();
+        if (!integration->hasCapability(QPlatformIntegration::BackingStoreStaticContents))
+            qWarning("QBackingStore::setStaticContents(): Platform does not support static contents");
+        return true;
+    }();
+
     d_ptr->staticContents = region;
 }
 

src/plugins/platforms/windows/qwindowsintegration.cpp

@@ -286,6 +286,8 @@ bool QWindowsIntegration::hasCapability(QPlatformIntegration::Capability cap) co
         return true;
     case SwitchableWidgetComposition:
         return false; // QTBUG-68329 QTBUG-53515 QTBUG-54734
+    case BackingStoreStaticContents:
+        return true;
     default:
         return QPlatformIntegration::hasCapability(cap);
     }

src/widgets/kernel/qwidgetrepaintmanager.cpp

@@ -706,7 +706,9 @@ void QWidgetRepaintManager::paintAndFlush()
 
     const QRect tlwRect = tlw->data->crect;
     if (!updatesDisabled && store->size() != tlwRect.size()) {
-        if (hasStaticContents() && !store->size().isEmpty() ) {
+        QPlatformIntegration *integration = QGuiApplicationPrivate::platformIntegration();
+        if (hasStaticContents() && !store->size().isEmpty()
+            && integration->hasCapability(QPlatformIntegration::BackingStoreStaticContents)) {
             // Repaint existing dirty area and newly visible area.
             const QRect clipRect(QPoint(0, 0), store->size());
             const QRegion staticRegion(staticContents(nullptr, clipRect));
@@ -1140,11 +1142,7 @@ void QWidgetRepaintManager::removeStaticWidget(QWidget *widget)
 
 bool QWidgetRepaintManager::hasStaticContents() const
 {
-#if defined(Q_OS_WIN)
     return !staticWidgets.isEmpty();
-#else
-    return !staticWidgets.isEmpty() && false;
-#endif
 }
 
 /*!

tests/auto/gui/kernel/qbackingstore/tst_qbackingstore.cpp

@@ -4,6 +4,8 @@
 #include <qwindow.h>
 #include <qbackingstore.h>
 #include <qpa/qplatformbackingstore.h>
+#include <qpa/qplatformintegration.h>
+#include <private/qguiapplication_p.h>
 #include <qpainter.h>
 
 #include <QTest>
@@ -276,9 +278,9 @@ void tst_QBackingStore::flush()
 
 void tst_QBackingStore::staticContents()
 {
-#if !defined(Q_OS_WIN)
+    const auto *integration = QGuiApplicationPrivate::platformIntegration();
-    QSKIP("Platform does not support static backingstore content");
+    if (!integration->hasCapability(QPlatformIntegration::BackingStoreStaticContents))
-#endif
+        QSKIP("Platform does not support static backingstore content");
 
     QWindow window;
     window.create();

tests/auto/widgets/kernel/qwidgetrepaintmanager/tst_qwidgetrepaintmanager.cpp

@@ -11,7 +11,9 @@
 #include <private/qhighdpiscaling_p.h>
 #include <private/qwidget_p.h>
 #include <private/qwidgetrepaintmanager_p.h>
+#include <qpa/qplatformintegration.h>
 #include <qpa/qplatformbackingstore.h>
+#include <private/qguiapplication_p.h>
 
 //#define MANUAL_DEBUG
 
@@ -429,6 +431,10 @@ void tst_QWidgetRepaintManager::opaqueChildren()
 */
 void tst_QWidgetRepaintManager::staticContents()
 {
+    const auto *integration = QGuiApplicationPrivate::platformIntegration();
+    if (!integration->hasCapability(QPlatformIntegration::BackingStoreStaticContents))
+        QSKIP("Platform does not support static backingstore content");
+
     TestWidget widget;
     widget.setAttribute(Qt::WA_StaticContents);
     widget.initialShow();