That's right. I can't drag the puck, if the cursor is at the edge of the screen. It even seems that clicking on the very right (2 pixels or so) of the puck doesn't allow you to drag it. By having the 'hitbox' of the puck at the edge of the screen, it makes it effectively infinitely wide and impossible to miss. If you're using a trackpad or have shaky hands it's an important difference.
(The compression on the gif makes it grainy, but you should be able to zoom in on the right and see what I mean.)
This is on Unity desktop, minimap disabled, most up to date version as of the time in the gif.