bug#50256: thing-at-mouse

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 50256@debbugs.gnu.org, larsi@gnus.org
> From: martin rudalics <rudalics@gmx.at>
> Date: Fri, 3 Sep 2021 09:40:36 +0200
> 
> If in the manual we say "The argument POSITION defaults to the current
> position of point in WINDOW", this means that the window should win.
> Whether that's reasonable is another question.  Note that a similar
> disputable case already happens when we do
> 
> (pos-visible-in-window-p nil (next-window) t)
> 
> and the next window does not show the current buffer.  We there silently
> override the "nil" with WINDOW's point.  Maybe we should signal an error
> in either of these cases when WINDOW's buffer is not the current buffer.
> 
> IIUC we use `pos-visible-in-window-p' mainly for three purposes:
> 
> (1) Detect whether a buffer position that typically does _not_ equal
>      window point is visible in window and, if it isn't, do something
>      (scroll, enlarge the window) to make it visible.
> 
> (2) Detect whether window point is only partially visible.
> 
> (3) Get the coordinates of window point and move the mouse or pop up a
>      menu there.
> 
> In all these cases, callers simply don't care about which buffer is
> current when calling the function - the buffer in question is WINDOW's
> buffer.
> 
> A different use were to check whether a position of an arbitrary BUFFER
> would be visible in a WINDOW if BUFFER were displayed there with the
> start of the window set to some valid BUFFER position.  I don't know
> whether anyone ever used such a functionality.  To make it work, a
> caller would have to set WINDOW's buffer and start position first, call
> `pos-visible-in-window-p' and restore the original state afterwards.
> Even in this hypothetical case, the caller wouldn't care about which
> buffer is current.

Thanks.  There's one more use case I can think of: when WINDOW is not
a selected one, but its buffer is also displayed in the selected
window, which could mean its point is different from WINDOW's point.

Anyway, after thinking for some time about this, I concluded that the
only sane way forward, especially since we are going to cut the
emacs-28 branch soon, is to leave the default behavior of
pos-visible-in-window-p and posn-at-point as it is today, and add one
more optional argument to force the possible alternative behavior(s).
The proposed change to event-start and event-end are new code, so they
should have no trouble passing this new optional argument to
posn-at-point.

That means to add an argument to pos-visible-in-window-p that would
cause it to select one of the following 3 alternatives: WINDOW's
point, WINDOW's buffer's point, and (in case WINDOW is the selected
window) the current buffer's point.  The default should stay as it is
today: when WINDOW is the selected window, use the current buffer's
point.

Anything else IMNSHO risks introducing many bugs into existing
well-tested code that we will never be able to discover and fix in
time for Emacs 28.1 release.