> 1. Open any text file, preferably with long lines (to better see the effect).
 >
 > 2. `M-x visual-line-mode RET`
 >
 > 3. `M-: (set-window-margins (selected-window) 0 150) RET`
 >
 > The number 150 is arbitrary. It's appropriate for my case (with a frame width of
 > about 238 characters).
 >
 > 4. `M-: (set-window-parameter (selected-window) 'min-margins '(0 . 0)) RET`
 >
 > 5. `C-x 3`
 >
 > At this point, I expect the window to be split into two equally-sized
 > side-by-side windows, but the split is at about one third of the original window
 > width.

The problem is that when we split the original window, the new window
does not get its 'min-margins' parameter set and the subsequent
sanitizing step screws up things.  Please try the attached patch.

And please look also into what happens when you shrink a frame with such
large margins to a size that won't accommodate those margins any more
unless they get shrunk.  I'm afraid that the subsequent sanitizing step
might cause troubles there as well.

Thanks, martin