** Changed in: canonical-devices-system-image
       Status: Fix Committed => Fix Released

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  Unity8 not informing clients that they are not visible and thus can
  stop rendering

Status in Canonical System Image:
  Fix Released
Status in QtMir:
  Fix Released
Status in unity8 package in Ubuntu:
  Fix Released

Bug description:
  Currently on the phone, apps stop rendering when shell hides them,
  because shell also lifecycle stops the app. App isn't actually told it
  should stop rendering, we just SIGSTOP its process.

  But some apps are not lifecycle stopped, e.g. music. So we should have
  unity8 tell the app to stop rendering.

  This will be a multi-step process:
  1. Qtubuntu
  Mir has a attribute to set on surfaces to tell them they are 
visible/occluded: (see mir/include/common/mir_toolkit/common.h)
  mir_surface_attrib_visibility - part of the MirSurfaceAttrib enum. 

  This indicates these possible values:
  typedef enum MirSurfaceVisibility
      mir_surface_visibility_occluded = 0,
  } MirSurfaceVisibility;

  On the client side (qtubuntu), you need to listen for this attribute change, 
and connect it to QWindowSystemInterface::handleExposeEvent() which tells Qt 
how much of a QWindow is being drawn.
  Note there is also a QWindowSystemInterface::handleWindowStateChanged() 
handler, which use use to explicitly set QWindow show/hide, but Qt tends to 
release its GL context on hide, which we may not desire. It may not hurt, worth 
a look.

  2. QtMir
  There are methods on mir::scene::Surface to set and get the MirSurfaceAttrib 
attributes. I think it would make sense to use the pre-existing 
MirSurfaceItem::visible property, so that if a surface is marked visible=false, 
then that will dispatch the mir_surface_visibility_occluded event to the 
client, so it will stop drawing.

  3. Unity8
  Correctly set visible=false when an application surface is occluded, and true 
when visible. QML has no automatic way of doing that (i.e. occlusion detection)

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