Video covers native child widget overlay when using OpenVG graphics system

It should be possible to place an overlay on top of video content using a widget hierarchy as follows:

TLW
|
+--Container
   |
   +--VideoWidget
   |
   +--Overlay

VideoWidget can either be:

• Derived from Phonon::VideoWidget, in which case the Phonon backend internally creates a child widget can calls winId() on it
• Derived from QWidget, in which case winId() is called by the VideoWidget itself and the native window handle passed to CVideoPlayerUtility2

The position and size of Overlay is updated whenever the position or size of VideoWidget changes - this can be done by installing an eventFilter on VideoWidget.

The z-order of Overlay is higher than that of VideoWidget.

Because winId() is called on either VideoWidget or one of its children, native window handles are created for all widgets in the tree, including Overlay. Video is therefore rendered into a separate native window from that used to display the overlay.

Environment

Qt: Built from source, qt-s60-public/4.7 commit 700ba679
Device: N8, wk26 RnD image

Steps to reproduce

This can be reproduced using the attached test app videooverlay_QTBUG-12607_1.zip.

The attached logs were created in the above environment using the following command line options [1]:

native.log

videooverlay.exe -container -video native -overlay nativechildwidget -graphicssystem openvg 

phonon.log

videooverlay.exe -container -video phonon -overlay nativechildwidget -graphicssystem openvg

Expected behaviour

Overlay is visible on top of video content [2].

Observed behaviour

1. When the application starts up, the UI is rendered correctly (other than the positioning error tracked by QTBUG-12605).
2. When video starts playing, it covers the overlay [3]

Notes

[1] When using the raster paint engine, native video rendering works correctly, but only if a special paint mode is set on VideoWidget:

videooverlay.exe -container -video native  -videowidgetpaint disable -overlay nativechildwidget -graphicssystem raster

or

videooverlay.exe -container -video native  -videowidgetpaint zerofill -overlay nativechildwidget -graphicssystem raster

Setting either paint mode when using OpenVG graphics has no effect.

[2] The overlay will be displayed transparent, even if Qt::WA_TranslucentBackground is set on it. This is because, to be shown translucent, the overlay must be a TLW, because this is the only way for the overlay to be rendered to a backing store which has an alpha channel. This can be done by running the test app with the following options:

videooverlay.exe -container -video phonon -overlay toplevelwidget -overlaypaint translucent -graphicssystem raster

The video and overlay are then rendered correctly (other than the positioning error tracked by QTBUG-12605).

[3] Whenever '-video native' is specified, the borders above and below the video content are filled with white, rather than the expected black borders.

[4] This is not a window z-ordering problem: calling raise() on Overlay, at the point when video is displayed, does not have any effect.