bug#29233: Enable fringe cursor when *almost* exact_window_width_line_p

[Keith David Bershatsky <esq@lawlist.com>]

The crux of the issue raised with #29233 is whether users might wish to see a fringe bitmap instead of a partial width cursor when point is slightly less than the exact window width.  Whereas the patches relating to #16856 reduce the width of the cursor so that it does not overflow into the fringe (creating unwanted artifacts), there was nothing done to permit a fringe bitmap to be placed there instead.

I had not given any consideration to the possibility that a partial width cursor is "a feature".  I.e., I erroneously assumed that a partial width cursor was merely an attempt to avoid unwanted artifacts being drawn on the fringe.

I personally like to see fringe bitmaps when points is slightly less than the exact widow width; however, I am uncertain how to reconcile that proposed feature with the desire by others to see a partial width cursor without a fringe bitmap.

If the consensus is that a user who has a window-width that is not an integral multiple of frame's character width must give up the ability to see a fringe bitmap, then that resolves #29233.

If the consensus is that a user does not have to give up seeing a fringe bitmap in the above-mentioned circumstance, the question is when will the fringe bitmap be seen and what modification would be appropriate in that circumstance.

Keith

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DATE:  [11-09-2017 23:53:56] <10 Nov 2017 09:53:56 +0200>
FROM:  Eli Zaretskii <eliz@gnu.org>
> 
> > Date: Thu, 9 Nov 2017 22:11:42 +0000
> > From: Alan Third <alan@idiocy.org>
> > * * *
> > 
> > Additionally I've attached a screenshot of the cursor in the fringe
> > when it shouldn't be. It should appear over the last x on the line
> > (point is before the last x on the line).
> 
> What you show is not cursor on the fringe, because you have the
> continuation arrow on the fringe.  When the continuation arrow is
> shown, the cursor cannot be shown on the fringe, because that slot is
> already taken.  And anyway, the cursor is only shown on the fringe
> when the line is not continued.
> 
> What you see there is Emacs displaying the small part of the cursor
> that it still has available on the first screen line, probably because
> your window-width is not an integral multiple of frame's
> character-width.
> 
> IOW, I don't think I see anything abnormal in that image.