when adjusting one point of line the other point flies away & all the other elements move

I'm using a line to indicate the voice crossing- the tenor voice goes over the alto, from the bass clef to a teble clef, so I'm wanted to use a line to indicate the voice crossing. However:
(1). when I adjust one point of a line, the other point of the line runs away - it automatically changes shape , and the other point changes the point of attachment.
(2). depending on the size of the line, the distance between treble and bass staffs in a grand staff changes.

I do not want this to happen, and I want the line to be a single independent line, just like a svg image that I can adjust.
is there any way to disable the automatic placement of the lines?
am I not using the right symbol for voice crossings?