I think that are good ideas.

Regarding the double click. I would even visualize the livewire always as a close contour, so always adding a straight line from current point to start point. Then you directly see who it looks like if you double click.
(And it will be closer to the appearance/behavior of the proposed new add tool)

Pushed new branch to rMITK MITK: feature/T28607-live-wire-improvements.

The listed changes have been implemented. I realized that there is an issue with the automatically created "last segment". If I click there to create a new control point and move it, I get the behavior shown in the image (sorry, I tried to explain it in words but failed).

The green point is moved. The expected behavior is that the contour between the moving point and the neighboring control points is recalculated.

The issue seems to be located in ContourModelLiveWireInteractor::SplitContourFromSelectedVertex. The behavior was always "wrong", but it was not that prominent due to the user always finishing the contour by clicking on the initial point.

@floca, @kislinsk: Both @kahl and @kleina won't be able to work on this anymore, but this task is still open. We also received some feedback regarding the usability from the last test day, so we should discuss how to proceed:

The first two comments should be targeted for this release. The last comment is something that also happens sometimes with the new implementation of the polygon tool that uses the same base class. But fixing is too complex to be worth spending time before the release, so whoever grabs this task, focus on the first two comments.

Regarding the comment of closing the contour with a double click: I also realized this during my work on the new add tool and I think the way I fixed it was to modify the OnFinish() function in the mitkLiveWireTool2D to check whether the click was performed on the first point which was also how it was done before the closure contour was added to the LiveWire tool.
Here is the code of my new add tool branch where I changed it:
https://phabricator.mitk.org/source/mitk/browse/feature%252FT28464-new-add-tool-2/Modules/Segmentation/Interactions/mitkLiveWireTool2D.cpp$344

Basically it is just the check

if (!OnCheckPoint(interactionEvent))
{
  m_Contour->Concatenate(m_ClosureContour);
}

I am not 100% sure if that is really all that needs to be done but maybe it helps as a starting point :)

@kahl So are you working on this? If not, could you remove yourself as the assignee of this task?

Deleted branch from rMITK MITK: feature/T28607-live-wire-improvements.