Comment 14 for bug 1385292

I was also having this issue. For me, the solution of setting "Keyboard input method system" to "None" ( System Settings -> Language Support -> Language tab) offered by dess seems to be fix. Thanks dess!

Still, I want to add a bit more information:

When my keyboard input method system was set to IBus, I had this problem both on 14.10 and on 15.04. On 14.10, I had this problem when using the Nvidia 331 driver from the repository and when using the Nvidia 346 driver from the ppa:xorg-edgers/ppa. In the case of 15.04, I also had this problem when using the Nvidia 331 driver from the repository and when using the Nvidia 346 driver from the ppa:xorg-edgers/ppa.

However, I have NEVER had this problem (whether or not my keyboard input method system was set to IBus) on on 14.10 and on 15.04 when using the NOUVEAU driver. Granted, I only tested for a short while with the nouveau driver, so I can't say for certain that the nouveau is immune to the problem.

QUESTION: Has anybody experienced this bug when using the NOUVEAU driver?

If not, maybe the what we're seeing is an interaction between the Nvidia driver and IBus.