> From: JD Smith <jdtsmith@gmail.com>
> Date: Thu, 24 Nov 2022 16:42:29 -0500
> Cc: emacs-devel@gnu.org
>
> Thanks. The documentation on posn-object-* provides no guidance that a
> âstring objectâ would not include a
> stretched string with specified space. So it was not at all surprising to me
> that it worked "as expected" in the
> GUI. In further support of this impression (leaving aside specified space),
> posn-object-x-y works perfectly
> well on normal width characters in the buffer, providing pixel level
> information on where *within the
> character* you clicked. In fact I cannot think what "pixel-based x and y
> coordinates relative to the upper left
> cornerâ would mean other than this for a "string object". Surely that is not
> an accident of the
> implementation!
Yes, but on TTY frames the DX/DY _within_ the character glyph are always
zero, because each character is considered to be exactly one "pixel". And
if I tell you that stretches of whitespace produced by 'space' display specs
are implemented on TTY as sequences of SPC characters (of course, what
else?), then I think you will understand why the DX/DY values, defined in
the ELisp manual as "the coordinates relative to the top left corner of the
character glyph clicked on", on a TTY are always zero: wherever the click
is, it is always on a glyph of some SPC character from those that represent
the stretch of whitespace.
> But it sounds like internal position X/Y offsets for specified spaces in GUI
> vs. TTY are not reliable, so maybe
> I should stop using them.
On TTY, you will always get zero DX/DY, so if this is deemed "unreliable",
then yes, you should not use that for your needs, whatever those are.
> Is there a reliable way to determine where (in pixels [1]) the mouse was
> clicked relative to a string displayed
> with specified space, which works equally in the GUI and TTYs? Keep in mind
> that the string may not be at
> a fixed column, but at an arbitrary pixel offset from the window (due to
> prior specified space, variable width
> fonts, etc.).
Please tell more about your use case. I don't think I understand what you
mean by "string displayed with specified space"; AFAIU there were no display
or overlay strings in the recipe you posted, and no description of what you
are trying to do with the 'space' spec that the DX/DY offsets are so
important.
If I have to guess what you want, then my suggestion would be to use
something like this:
(let* ((event-posn (posn-point (event-start ev)))
(click-x-y (posn-x-y (event-start ev)))
(obj-x-y (posn-x-y (posn-at-point event-posn))))
(cons (- (car click-x-y) (car obj-x-y))
(- (cdr click-x-y) (cdr obj-x-y))))
IOW, don't trust DX/DY, but calculate the offsets "by hand".
But that's a guess.
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