() Eli Zaretskii <eliz@gnu.org>
() Thu, 16 Oct 2014 10:21:52 +0300

   why we subtract 1 pixel from the cursor glyph's pixel_width

I see in XDrawRectangle(3):

 The XDrawRectangle and XDrawRectangles functions draw the
 outlines of the specified rectangle or rectangles as if a
 five-point PolyLine protocol request were specified for
 each rectangle:

   [x,y] [x+width,y] [x+width,y+height] [x,y+height] [x,y]

So if you use the glyph width directly, the resulting rectangle
drawn will invade the next character cell by one pixel.  E.g.:
Given a glyph width of ten, this is wrong:

 | 0         1         2
 | 012345678901234567890123456
 |           XXXXXXXXXXX
 |           X         X

but this is right:

 | 0         1         2
 | 012345678901234567890123456
 |           XXXXXXXXXX
 |           X        X

Probably (i don't have docs handy) the other toolkits'
rectangle-drawing primitives draw from X,Y to X+W-1,Y+H-1
(NB the -1) so there is no need to manully decrement for them.

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