>   (display-monitor-attributes-list)
>   ;; ==>
>   '(((geometry 0 0 1920 1080)
>      (workarea 0 0 1920 1050)
>      (mm-size 677 381)
>      (name . "\\\\.\\DISPLAY1")
>      (frames ...))
>     ((geometry -1680 -1050 1680 1050)
>      (workarea -1680 -1050 1680 1050)
>      (mm-size 593 370)
>      (name . "\\\\.\\DISPLAY2")
>      (frames ...)))

I see that ‘geometry’ is the same as ‘workarea’ on the second monitor,
but slightly different on the first one.  Mine are all the same,
so I don't know whether to use ‘geometry’ or ‘workarea’.

Meanwhile I found a mistake in my previous patch.  I assumed that
both pairs of attributes top/left and height/width for ‘make-frame’
are measured in pixels.  In fact, only top/left are in pixels,
whereas height/width are in text units.  So needed to wrap them
in ‘text-pixels’.  Now with this patch everything is fine: