The following is a part of an QtWayland example:

WaylandOutput {
        sizeFollowsWindow: true
        window: Window {
            width: 1024
            height: 768
            visible: true

            Repeater {
                model: shellSurfaces

                ShellSurfaceItem {
                    shellSurface: modelData
                    onSurfaceDestroyed: shellSurfaces.remove(index)
                }
            }
        }
}


If I have two displays, and a part of a ShellSurfaceItem is in the first 
display, and the other part of the ShellSurfaceItem in the second display. In 
this case, I need to ensure the user can see this ShellSurfaceItem
on both first and second displays, on QtWayland, I can using two WaylandOutput 
with two Window for this case, and create ShellSurfaceItem for which 
WaylandOutput.

But, If I using two ShellSurfaceItem for one wl_surface, I can't sync the 
window animation in different WaylandOutput, this is a hard job, because the 
QQuickItem must bind to single Window, it's can't render to multi Windows.

The better way is redesign QtQuick scene graphics, like as:


  1.  Add QQuickScene class to instead of a part of functions of QQuickWindow. 
a QQuickItem must be added to a QQuickScene, a QQuickScene is a coordinate 
system.
  2.  Add QQuickViewport class, and allow attach multi QQuickViewport to one 
QQuickScene. the QQuickViewport be responsible for render QQuickItem to target 
output. And allow the QQuickViewport can render in a new thread(be different 
than the other QQuickViewport in same QQuickScene).
  3.  Add QQuickRenderTarget::fromSurface(EGLSurface/VkSurface) functions, and 
using QQuickRenderTarget to QQuickViewport::render(QQuickRenderTarget target).
  4.  The QQuickWindow don't depends on QSG* class, it can wapper 
QQuickViewport and QQuickRenderTarget to render QQuickItem.
  5.  Add QQuickSceneHelper abstract class, The QQuickWindow event dispense to 
QQuickSceneHelper, and QQuickSceneHelper dispense event to QuickItem. Provide 
like as "virtual QQuickSceneHelper::setCursor" functions to instead of 
QQuickWindow::setCursor in QQuickItem::setCursor.
  6.  The QQuickItem without QQuickWindow, using QQuickItem::pixelRatio to 
instead of QQuickWindow::effectiveDevicePixelRatio(), the 
QQuickItem::pixelRatio is only using for load resources(like as 
QQuickImage::load), for the render matrix, using the 
QQuickViewport::devicePixelRatio when rendeing.

The above changes may not change the abi.

If I misunderstand QtWayalnd and QtQuick Scene Graphics, please tell me, thanks.


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