I discovered that in OS 10.7.5 when no audio Output device is selected, I could not use the mouse to move objects on a canvas. Once I selected an audio Output device, my ability to move objects with the mouse was restored. In either case, I could select objects and move them with the arrow keys.
You can try to duplicate this problem by turning off Output device 1 in Audio Settings.
Anonymous
I couldn't reproduce this (vanilla Pd 0.43) - what Pd distro audio hardware
are you using?