Hi Marc, there are actually 3 ways to finish the polyline - you can press the Done button, right-click in a viewport, or press the Enter key on the keyboard.

Normally canceling a command will also erase the object being currently constructed - this is something that is a uniform behavior to every command, and in this case Escape is an alias for pressing the "Cancel" button.

But it is possible to customize the polyline command to do what you want though - download the attached DoCurve.zip file, unzip it to get DoCurve.js, and then copy this over top of the old version inside of the \commands subfolder underneath MoI's main installation folder under c:\Program Files.

This customized version will finalize the polyline when you press escape, and the same thing will apply to the Freeform / Control points and Freeform / Through points commands as well.

Let me know if you have any problems with it.

- Michael