Unnecessary creation of decoration on certain circumference

1. preconditions

QSG_RENDER_LOOP=threaded

QT_WAYLAND_SHELL_INTEGRATION=ivi-shell

setPersistentOpenGLContext(true)

setPersistentSceneGraph(true)

2. short description about this issue

Black screen occurs when app transitions from foreground -> background -> foreground again. (by using hide/show call)

3. Detail call sequence

When the app transitions from foreground to background by using hide()

void QWaylandWindow::setVisible(bool visible)
-> QWaylandWindow::reset() 
   at this point, delete mShellSurface
Then, 
QSGRenderThread::invalidateGraphics() is called
   at this point, current = gl->makeCurrent(fallback ? static_cast<QSurface *>(fallback) : static_cast<QSurface *>(window)); called
-> bool QWaylandGLContext::makeCurrent(QPlatformSurface *surface) 
-> window->createDecoration();
in this function, decoration will be created by "if (mShellSurface && !mShellSurface->wantsDecorations())" statement because mShellSurface was deleted already.
after that
FBO (Frame Buffer Object) for decoration is binded to the default frame buffer by below code in QWaylandGLContext::makeCurrent()
QOpenGLContextPrivate::setCurrentContext(context());
window->bindContentFBO();

Later, when the app switches background->foreground by using show()

Black screen occurs because the default frame buffer is still exist.

3. Possible solution

We can avoid this problemm by setting 

QT_WAYLAND_DISABLE_WINDOWDECORATION=1

and also we don't have to ship the decoration plugin to the target board.

4. Expectation

However, creation of decoration in this circumference is unnecessary because we are using ivi-shell. So I guess there may be improvements in qtwayland.

*. This issue has been reported by a customer.