Invisible mouse in 15.04

[Grokling]

kubuntu 15.04 x64 freshly installed from ISO
Two NVIDIA GT210 cards, two panels on each.

The system starts, logs in, and brings up all four of my screens (which is fantastic right out of the box using the default Nouveau drivers - no other distro has managed this for me yet)

BUT, I have an invisible mouse cursor. It works, but I have to guess where it is. It doesn't seem to be affected by changing to a different theme.

The NVIDIA driver seems to break things so that xrandr can only see one gpu (so only two screens work with that driver)
I can use nvidia-settings to bring up the other two, but then I have to use xinerama to get them to work properly, and that breaks other things.

I tried these things, they didn't work, so I reinstalled to clean it up. But, still no mouse.

The strangest thing is I could swear that I had a mouse working the first time I installed this ISO.

[Grokling]

kubuntu without xorg.conf snippets: works but has no visible mouse cursor.
kubuntu with an xorg.conf snippet that disables HWCursor: kdm works, desktop session crashes.

[Grokling]

Okay. Interesting. It seems to be an issue with compositing.
system settings - display - compositing tells me it's using opengl2 as it's default.

IF I change that to xrender, I get a mouse cursor back.

Once I've done that, I can change it to opengl2 or opengl3.1, hit apply, and still have a mouse cursor.
Until I log out and back in..

if I leave it with xrender as the rendering backend, it works, and stays working (so far anyway)

This seems like a bug, but I don't know how to raise it as such, or what details are useful. Happy to dump logs etc if someone can guide me on what is needed.