When previewing a Blend Tree, clicking the red line (blend control red line in the Inspector preview) makes the red line jump to the right for no reason. You can then drag the red line to preview the blend, but notice the offset between the mouse and the red line itself, created by the offset jump when you first clicked the line.

repro:
-open project
-open Set-up scene
-open the Animator window
-select mangagirl in the Hierarchy
-double click Blend Tree and in the Inspector...
-click and drag the red line...
result: offset on click, then offset between mouse and line stays during drag.
expected result: red line does not move on click, then moves with the mouse.