> I changed <= to < to fix only Y coordinates for top/bottom windows,
 > but maybe X coordinates should still use <= ?

The function has to treat the case where an edge is shared between two
windows separately.  A non-shared edge always belongs to the
corresponding window.

 > Please note that the crucial difference is that now it uses
 > frame relative positions rather than window relative ones
 > with '(window-edges window nil nil t)' of the window to be deleted,
 > and window coordinates added to X and Y of posn-at-point.

Right.  But the X and Y values are needed separately only at the time
`window-at-pos' is called.

 > 1. `get-mru-window' could be one possible choice of the new option.
 > 2. `use-posn-at-point' could be another choice.

I called them just 'mru' and 'pos' now.  The option is called
`selected-window-after-deletion', there might be better names.

 > 3. Selecting the first window could remain in `delete-window-internal'.

It has to stay there.

 > 4. But what to do with this code block with '(other-window -1 frame)'?
 >     Should this be simply deleted?

That `other-window' was a bug: A frame that is not selected should not
get selected because we delete a window on it.  In addition, the old
code did not record a newly selected window unless that window was
produced by `get-mru-window'.  I tried to fix those too.  Please have a
look.

martin