Ubuntu'sin this month's Ubuntu release have been point on hold after issues with its X server compatability layer, XMir, could not be ironed out in time.
In an update delivered to the Ubuntu development mailing list, Canonical director of product strategy engineering Oliver Ries said that issues with XMir's multi-monitor support prevented its use by default in Ubuntu 13.10.
"Mir has made tremendous progress and is currently available on the Ubuntu archive for use, but there are still some outstanding quality issues that we want to resolve before we feel comfortable turning it on by default," Ries said.
"Many of these issues live in the XMir part of the stack, which provides the integration between the X server and the underlying Mir system compositor. More specifically, the multi-monitor support in XMir is working, but not to the extend [sic] we'd like to see it for all of our users."
Coming into its 13.10 development cycle, Ubuntu had aimed up at delivering a Mir, XMir, and Unity 7 stack for desktop users, and Mir with Unity 8 for users running Ubuntu Touch. Ries said that they would only be able to default Mir with the Ubuntu Touch images.
"The core of Mir is working reliable, but with XMir being a key component for our 13.10 goals, we didn't want to compromise overall Ubuntu quality by shipping it," he said.
The decision pulls Ubuntu into line with its derivative distributions that have already signalled their intentions not to follow the parent Linux distribution's decision to support Mir by default.
Kubuntu has said that it will remain on X and hopes to move to Mir's better supported alternative Wayland, while Xubuntu said it would not support Mir in any 13.10 release, LXDE-based Lubuntu looking to stick with X for 13.10 and 14.04 release.
Intel has also withdrawn support for XMir stating that: "We do not condone or support Canonical in the course of action they have chosen, and will not carry XMir patches upstream."
In November 2010, Canonical founder Mark Shuttleworth announced that Ubuntu would follow the rest of the Linux ecosystem would make the transition over to Wayland.
"Progress on Wayland itself is sufficient for me to be confident that no other initiative could outrun it, especially if we deliver things like Unity and uTouch with it," Shuttleworld said.
At the start of this year though, Canonical had reversed its decision and thrown its resources behind creating the Mir display server for its desktop and phone distribution.
Ubuntu 13.10 is due to be released on October 17.