It is hard to select single-segment horizontal or vertical lines

[iwbnwif]

This is because the first test in wxSFLineShape::GetHitLinesegment is whether the point is within the line's bounding box.

Given that a horizontal line has a bounding box of 2px height, it requires very accurate mouse placement.

Suggest to inflate the line bounding box before testing for "Contains".

    wxRect wholeLineBB = GetBoundingBox();
    wholeLineBB.Inflate(3);

    if(!wholeLineBB.Contains(pos))
        return -1;