Thanks for review, @halton.
The only purpose of `if (0 != physical_width_ && 0 != physical_height_)` condition is to avoid division by zero.  I am not sure whether `physical_xxx` equal 0 for some special scenarios. Maybe when failed to update them by calling `DisplayxxxMM(dpy, DefaultScreen(dpy))` for the first time, then `physical_xxx` keep their default value(0.0).