Window focus constantly switching really fast between sidebar and window

[saintwolf]

Not sure if it is a setting that I have enabled, but every now and then when I return to Chrome, it will keep switching focus back and forth really fast between the sidebar and the window to which it is docked.

I've had a look at the console whilst it happens and I can't see any output, so will need to insert some console.log()s to see where this is happening.