You might try using the escape key, which will stop the current trace. Escape, double-click, Escape, double-click, etc.
At one point in the distant past, double-clicking the history list would change the target, too. It turned out that it was a bit *too* easy to trace a new target, and people (ie: me) were occasionally tracing to a new target and losing all the history on the existing target. You obviously thought about this since you advocated the "prompt for save" portion of this.
Give the "Escape, double-click" a whirl and let us know what you think. If that doesn't work for you, we'll see if we can figure out a good solution that is fast and easy, but not risky for data loss.
The icon change has been a bit freaky. Maybe we should have saved it for an April 1st switch!