widgets: Invalidate RHI swapchain when window moves to new top level

When a QWidget with an associated RHI swapchain (via its QWindow) is
moved to a different top level window, that top level window has its
own backing store, and QBackingStoreRhiSupport, which doesn't know
anything about the fact that the window already has an associated
swap chain in the original top level window's QBackingStoreRhiSupport.

As having multiple swap chains for the same window is not supported
on all RHI backends (Vulkan and DX in particular), we need to throw
away the swap chain when detecting that the window is moved to a new
top level.

We do this by hooking into the existing WindowAboutToChangeInternal
event delivery to renderToTexture children, which now delivers the
event both to renderToTexture QWidget children as well as QWindows
in the hierarchy. The condition of when to deliver the event has
been updated to reflect whether the top level uses RHI for flushing,
instead of only including renderToTexture children, as the former
also includes setting QT_WIDGETS_RHI=1 explicitly.

The event is then caught by QBackingStoreRhiSupportWindowWatcher,
and handled the same way as for SurfaceAboutToBeDestroyed.

Renaming qSendWindowChangeToTextureChildrenRecursively would make
sense at this point, but to make cherry-picks easier we keep the
current name for now.

Fixes: QTBUG-120276
Change-Id: Ic4c60e89be985f12a84e9f893c299e602b70851a
Reviewed-by: Qt CI Bot <[email protected]>
Reviewed-by: Laszlo Agocs <[email protected]>
(cherry picked from commit 1bd755465efa27294362925f55306f88f1800936)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>
(cherry picked from commit 81581819c79c9195b0a732cc3ba99a67a18c06f0)
Reviewed-by: Tor Arne Vestbø <[email protected]>

2 files changed, 13 insertions, 6 deletions

diff --git a/src/gui/painting/qbackingstorerhisupport.cpp b/src/gui/painting/qbackingstorerhisupport.cpp
index 422159dfd90..55cfb9a61bc 100644
--- a/src/gui/painting/qbackingstorerhisupport.cpp
+++ b/src/gui/painting/qbackingstorerhisupport.cpp
@@ -181,13 +181,14 @@ QRhiSwapChain *QBackingStoreRhiSupport::swapChainForWindow(QWindow *window)
 
 bool QBackingStoreRhiSupportWindowWatcher::eventFilter(QObject *obj, QEvent *event)
 {
-    if (event->type() == QEvent::PlatformSurface
-            && static_cast<QPlatformSurfaceEvent *>(event)->surfaceEventType() == QPlatformSurfaceEvent::SurfaceAboutToBeDestroyed)
+    if (event->type() == QEvent::WindowAboutToChangeInternal
+        || (event->type() == QEvent::PlatformSurface
+            && static_cast<QPlatformSurfaceEvent *>(event)->surfaceEventType() == QPlatformSurfaceEvent::SurfaceAboutToBeDestroyed))
     {
         QWindow *window = qobject_cast<QWindow *>(obj);
         auto it = m_rhiSupport->m_swapchains.find(window);
         if (it != m_rhiSupport->m_swapchains.end()) {
-            qCDebug(lcQpaBackingStore) << "SurfaceAboutToBeDestroyed received for tracked window" << window << "cleaning up swapchain";
+            qCDebug(lcQpaBackingStore) << event << "received for" << window << "- cleaning up swapchain";
             auto data = *it;
             m_rhiSupport->m_swapchains.erase(it);
             data.reset(); // deletes 'this'
diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index 0e7a86e4665..f78504b338f 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -10643,9 +10643,15 @@ void qSendWindowChangeToTextureChildrenRecursively(QWidget *widget, QEvent::Type
 
     for (int i = 0; i < d->children.size(); ++i) {
         QWidget *w = qobject_cast<QWidget *>(d->children.at(i));
-        if (w && !w->isWindow() && QWidgetPrivate::get(w)->textureChildSeen)
+        if (w && !w->isWindow())
             qSendWindowChangeToTextureChildrenRecursively(w, eventType);
     }
+
+    // Notify QWidgetWindow after we've notified all child QWidgets
+    if (auto *window = d->windowHandle(QWidgetPrivate::WindowHandleMode::Direct)) {
+        QEvent e(eventType);
+        QCoreApplication::sendEvent(window, &e);
+    }
 }
 
 /*!
@@ -10705,7 +10711,7 @@ void QWidget::setParent(QWidget *parent, Qt::WindowFlags f)
 
     // texture-based widgets need a pre-notification when their associated top-level window changes
     // This is not under the wasCreated/newParent conditions above in order to also play nice with QDockWidget.
-    if (d->textureChildSeen && ((!parent && parentWidget()) || (parent && parent->window() != oldtlw)))
+    if ((oldtlw && oldtlw->d_func()->usesRhiFlush) && ((!parent && parentWidget()) || (parent && parent->window() != oldtlw)))
         qSendWindowChangeToTextureChildrenRecursively(this, QEvent::WindowAboutToChangeInternal);
 
     // If we get parented into another window, children will be folded
@@ -10786,7 +10792,7 @@ void QWidget::setParent(QWidget *parent, Qt::WindowFlags f)
 
     // texture-based widgets need another event when their top-level window
     // changes (more precisely, has already changed at this point)
-    if (d->textureChildSeen && oldtlw != window())
+    if ((oldtlw && oldtlw->d_func()->usesRhiFlush) && oldtlw != window())
         qSendWindowChangeToTextureChildrenRecursively(this, QEvent::WindowChangeInternal);
 
     if (!wasCreated) {