Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

I have noticed this problem for some time. I wonder if there's any
possibility in fixing it. It is so noticeable that I have tried my best
not to use the maximisation button on the title bar as much as I can.

The maximisation button, the green button on the title bar, does not
perform maximising the window (frame in Emacs) properly. It leaves a
wide blank strip on top of the header. See this screenshot: