Weird behaviour from Opera Beta, it froze the Mac. I could select Force Quit but no apps would quit. I keep Activity Monitor in the dock but the dock was unresponsive. Forced shutdown, booted from Recovery drive & repaired disk.
Still misbehaving after reboot, entire desktop window moved 1 cm to the right. Forced shutdown again. This time I booted from a Yosemite clone on an ext SSD over Thunderbolt. DiskWarrior said the directory needed a rebuild, so I did that. Rebooted normally & now all is OK.