Make mode-line appear to be a line

[Yuan Fu]

I tried to hide mode-line, but then I can’t separate two windows splitter 
vertically (shown in the attachment). Is there a way to make mode-line appear 
as a line, or draw a line between buffers without mode-line? Or is this a Mac 
problem? (I tried (setq mode-line-format '(:propertize " " :height 1)), it 
doesn’t work, the mode-line still takes the full-height of a line.)

Thanks,
Yuan