I'm a pretty experienced user of Snap!
What is happenening when left click all of a sudden brings up a white blinking line. Any block you click on just has a solid white line that blinks at the top of the block. You can't select blocks any more after this. One way to stop it that just worked for me was right clicking and selecting add comment, then deleting the comment.
Is it some sort of turbo or debugging/stepping mode?