Maybe not the most inspiring headline but a useful hint for anyone using Paul Falstad's ripple tank. I've often struggled to draw a straight line with the boundary tool (the usual trick of holding shift doesn't work). I normally almost get to the end of the line before messing up.

Sina and Liv came up with a good method, use ruler. No, not on the screen , on the touchpad. You need 4 hands but it seems to work.