Turn off power management of bluetooth mouse (i.e. Microsoft Designer Mouse)

/etc/bluetooth/input.conf

# Configuration file for the input service

# This section contains options which are not specific to any
# particular interface
[General]

# Set idle timeout (in minutes) before the connection will
# be disconnect (defaults to 0 for no timeout)
#IdleTimeout=30
IdleTimeout= 0

# Enable HID protocol handling in userspace input profile
# Defaults to false (HIDP handled in HIDP kernel module)
#UserspaceHID=true

reboot.