why我的桌面會有一條直線?

This always is the monitor problem, if you open wndows (Not the desktop) but you still can find it, that the monitor have some problem.
This line is the space which is cannot be displayed, that is become unstable.
Try to replace another monitor. If the line still in there, check the display card.

我估係一條死線,你去換番個(mon)la