QOpenGLCompositorBackingStore: ensure backing store on flushed windows

When the first QEglFSWindow got created for a QWindow, a backing store
was created with it and associated to the new QEglFSWindow. When the
window was hidden on the platform screen, the QEglFSWindow got deleted
and re-created when it was supposed to be shown.
The re-created QEglFSWindow was no longer associated to the backing
store. It was therefore not rendered on the screen.

=> ensure that the backing store is re-associated to the QEglFSWindow,
corrsponding to the QWindow argument passed to flush().

No autotest is added, because the fix is purely visual.
The widgets/menus example can be used to test the functionality
manually (see bug reports).

Fixes: QTBUG-115196
Fixes: QTBUG-116769
Pick-to: 6.2
Change-Id: I92ce2e8f7e76bd2d26e713a4d20612d079fb4717
Reviewed-by: Qt CI Bot <[email protected]>
Reviewed-by: Eirik Aavitsland <[email protected]>
(cherry picked from commit 4bc8f548222a0dd2967063a23e76c37d302253cb)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>
(cherry picked from commit 4bc76c2647407ff3758d803a4b4c717699b5001a)

3 files changed, 8 insertions, 2 deletions

diff --git a/src/opengl/qopenglcompositor_p.h b/src/opengl/qopenglcompositor_p.h
index abe8d4959ef..66dd77c1c21 100644
--- a/src/opengl/qopenglcompositor_p.h
+++ b/src/opengl/qopenglcompositor_p.h
@@ -29,6 +29,7 @@ class QOpenGLFramebufferObject;
 class QWindow;
 class QPlatformTextureList;
 
+class QOpenGLCompositorBackingStore;
 class QOpenGLCompositorWindow
 {
 public:
@@ -37,6 +38,8 @@ public:
     virtual const QPlatformTextureList *textures() const = 0;
     virtual void beginCompositing() { }
     virtual void endCompositing() { }
+    virtual void setBackingStore(QOpenGLCompositorBackingStore *backingStore) = 0;
+    virtual QOpenGLCompositorBackingStore *backingStore() const = 0;
 };
 
 class Q_OPENGL_EXPORT QOpenGLCompositor : public QObject
diff --git a/src/opengl/qopenglcompositorbackingstore.cpp b/src/opengl/qopenglcompositorbackingstore.cpp
index a5cc391e48f..841087ed6ff 100644
--- a/src/opengl/qopenglcompositorbackingstore.cpp
+++ b/src/opengl/qopenglcompositorbackingstore.cpp
@@ -137,6 +137,9 @@ void QOpenGLCompositorBackingStore::updateTexture()
 void QOpenGLCompositorBackingStore::flush(QWindow *window, const QRegion &region, const QPoint &offset)
 {
     // Called for ordinary raster windows.
+    auto *handle = dynamic_cast<QOpenGLCompositorWindow *>(window->handle());
+    if (handle && !handle->backingStore())
+        handle->setBackingStore(this);
 
     Q_UNUSED(region);
     Q_UNUSED(offset);
diff --git a/src/plugins/platforms/eglfs/api/qeglfswindow_p.h b/src/plugins/platforms/eglfs/api/qeglfswindow_p.h
index d111042040a..e51cd69f3df 100644
--- a/src/plugins/platforms/eglfs/api/qeglfswindow_p.h
+++ b/src/plugins/platforms/eglfs/api/qeglfswindow_p.h
@@ -70,8 +70,8 @@ public:
     bool isRaster() const;
 
 #ifndef QT_NO_OPENGL
-    QOpenGLCompositorBackingStore *backingStore() { return m_backingStore; }
-    void setBackingStore(QOpenGLCompositorBackingStore *backingStore);
+    QOpenGLCompositorBackingStore *backingStore() const override { return m_backingStore; }
+    void setBackingStore(QOpenGLCompositorBackingStore *backingStore) override;
     QWindow *sourceWindow() const override;
     const QPlatformTextureList *textures() const override;
     void endCompositing() override;