Draw a straight line when rotating the canvas

I'm working on an complete tutorial about how to draw a line in Photoshop and I'm having a bit of trouble with the rotate canvas feature in Photoshop.

The rotate canvas feature is great if you want to draw straight lines especially if you don't have a graphics tablet. It's much easier to draw straight horizontal lines than diagonal.

My question is: when drawing a line with the Line Tool you can hold down the SHIFT key to snap to exact angles when drawing. When rotating the canvas though, why does Photoshop consider the horizontal/vertical of the canvas and not the entire window? Is there a setting to adjust this?

In the screenshot you can see a line that I tried to draw holding down SHIFT. I wanted it to be parallel to the screen horizontal. In stead it snapped to what I think is 45 degrees from the canvas horizontal.

thanks for any input on this,

John