Comment 24 for bug 641712

If you change /etc/kde4/kdm/kdmrc by setting these values, you can avoid this bug:

# Whether to grab keyboard and mouse while the greeter is visible. Grabs
# may improve security, but make on-screen keyboards, etc. unusable.
# "Never" - never grab
# "IfNoAuth" - grab if the display requires no X authorization
# "Always" - always grab
# Default is IfNoAuth
GrabInput=Always
# Hold the X-server grabbed the whole time the greeter is visible. This
# may be more secure, but it will disable any background and other
# X-clients started from the Setup script.
# Default is false
GrabServer=true
# How many seconds to wait for grab to succeed.
# Default is 3
GrabTimeout=3