Hi Armin, thanks for reporting this problem!

The problem in this case is that currently a circular arc is extended only by lengthening the arc.

So for example if you have an arc like this and a boundary:



If you use Extend on it, the arc will lengthen to a longer arc until it hits that boundary:



If the arc is too small in radius to hit the boundary then nothing will be generated for that case.

So that's why you don't get that kind of extension that you were looking for in this case. In the future I'd like to add some more options to Extend but for now you can get what you want by drawing a line. If you have "Straight Snap" and "Object Snap" enabled, you should be able to draw a line that has an intersection snap on the ending location where you want, like this:



I tried to replicate your trim problem over here with a line that was overshooting, and I could not seem to reproduce a problem there. It may have something to do with your particular shapes or something, are they on different elevations possibly?

If you can please post the .3dm model file with the curves in it that do not trim correctly, that would help me to be able to inspect your curves to see what is happening in that case.

- Michael