Love it! But I am having a hard time figuring out when exactly I can/can't "pick up" a point from somewhere else: sometimes tick marks appear at the distance I

Love it! But I am having a hard time figuring out when exactly I can/can't "pick up" a point from somewhere else: sometimes tick marks appear at the distance I'm looking for, sometimes not. Also, it seems odd that I can't just draw a line (as opposed to line segment), since iirc the straightedge is theoretically infinite. (There are definitely a few times I've wanted to lay down the full line traversing two points, for later use in the construction....) EDIT: it looks like the tick marks are the straight line extensions; if so, it's exceedingly strange that I can use them "for free" (without drawing the line) as, say, guides for the radius of a circle (I can't put the centre there, but I can "end" the circle there), but I can't just have the line itself.