DIAGNOSIS This is a physical connection issue, not a software or signal problem. The internal ribbon cable connecting the display panel to the controller board is becoming loose or partially disconnected when the monitor flexes.
FIX This requires addressing the physical connection. Gently press along the bottom bezel of the monitor, near where the line appears. Do not apply excessive force to the screen itself. If pressure on the bezel makes the line disappear, the connection is loose. For a permanent fix, you would need to open the monitor chassis and reseat the ribbon cable, which is not recommended for most users.
IF THAT DIDN'T WORK Test with a different video source and cable to completely rule out an external issue. If the line persists and is tied to movement, the diagnosis is confirmed.
HEADS UP This is a common point of failure on many portable monitors due to hinge stress and general flexing. Avoid putting pressure on the screen or folding it back too far, as this can worsen the connection.