I started looking into this and what I found inside mitk::RegionGrowingTool::OnMousePressed:

if (inside)
      {
        MITK_DEBUG << "Clicked inside segmentation";
        // For now, we're doing nothing when the user clicks inside the segmentation. Behaviour can be implemented via
        // OnMousePressedInside()
        // When you do, be sure to remove the m_PaintingPixelValue check in OnMouseMoved() and OnMouseReleased()
        return;
      }
      else
      {
        MITK_DEBUG << "Clicked outside of segmentation";
        OnMousePressedOutside(nullptr, interactionEvent);
      }

Looking at the file history it doesn't look like that was implemented in MITK at anytime.
So we can actually discuss if we want to allow such a functionality and if, what the obstacles are.

I quickly modified the code to see if this works potentially. For this I set m_PaintingPixelValue to 1 for both cases and used the OnMousePressedOutside-function also for a mouse pressed inside:

m_PaintingPixelValue = inside ? 1 : 1;

      if (inside)
      {
        MITK_DEBUG << "Clicked inside segmentation";
        // For now, we're doing nothing when the user clicks inside the segmentation. Behaviour can be implemented via
        // OnMousePressedInside()
        // When you do, be sure to remove the m_PaintingPixelValue check in OnMouseMoved() and OnMouseReleased()
        OnMousePressedOutside(nullptr, interactionEvent);
        return;
      }
      else
      {
        MITK_DEBUG << "Clicked outside of segmentation";
        OnMousePressedOutside(nullptr, interactionEvent);
      }

This probably needs to be tested thoroughly with 3D-/4D-Data, with static and dynamic segmentations and multi label. What do you think, @floca?

Hm, due to the facts that (1) the feature is missing since 2016 and no one ever complaint, (2) it is not documented in the help and (3) I am not sure why one would like to behave differently when clicking in an area that is segmented, I would just skip the code path. Yes.
But I would go further then. You don't need the whole inside check any more. I would completly remove everythin below line 332 and directly call OnMousePressedOutside(nullptr, interactionEvent); then.

This issue should be postponed until T28523: Evaluate MITK segmentation survey (to now the new priority) is done.

@floca, should we also add this for the upcoming release?

In T28320#229111, @kalali wrote:

@floca, should we also add this for the upcoming release?

Is there already an task branch ready to land? If not I think it is unrealistic to bring it in the next release, as either the release branch is already forked or will be any minute or so...
And I would not break the time based pattern.