Re: Moving point after character when clicking latter half of it

[Moritz Maxeiner <mm@ucw.sh>]

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On Sunday, 9 July 2023 16:14:06 CEST Eli Zaretskii wrote:
> > From: Moritz Maxeiner <mm@ucw.sh>
> > Cc: emacs-devel@gnu.org
> > Date: Sun, 09 Jul 2023 15:51:10 +0200
> > 
> > On Sunday 9 July 2023 15:23:40 CEST Eli Zaretskii wrote:
> > > That's not true.  buffer_posn_from_coords calls several functions of
> > > the move_it_* family, including move_it_in_display_line_to itself, and
> > > the information that your proposed patch accessed via it->pixel_width
> > > is available to buffer_posn_from_coords through the 'struct it'
> > > variable it passes to those move_it_* functions.  The only
> > > complication is that you might need to call these functions more
> > > times, to asses whether this or the next glyph are closer to the click
> > > coordinates.
> > 
> > Ok, thanks for the corection. I might look into this, then.
> 
> Basically, you need to examine it.current_x after the last call to
> move_it_in_display_line returns, and if it is closer to
> to_x+it.pixel_width than to to_x, call move_it_in_display_line again
> to get to to_x+it.pixel_width.

Thanks, I've implemented something along those lines. Same source position,
but the check is if to_x exceeds half of the selected display element's width
and if so rerun move_it_in_display_line at it.current_x + it.pixel_width
(so first pixel of the next display element).
Running it at to_x+it.pixel_width might run into trouble if the current display
element is a lot wider than the one after it.

I've also added an option mouse_point_alternate (temporary name, didn't know
what else to call it, open to suggestions) to enable it. It defaults to off.

> 
> > > > Would adding another option to move_it_in_display_line_to be acceptable?
> > > > That way only functions that explicitly select the new halfway behavior,
> > > > like I did with buffer_posn_from_coords in the new version of the poc
> > > > patch.
> > > 
> > > I don't see how this would work.  buffer_posn_from_coords calls
> > > move_it_to and move_it_in_display_line, it doesn't call
> > > move_it_in_display_line_to directly. 
> > 
> > It works because move_it_in_display_line forwards its op parameter to 
> > move_it_in_display_line_to. See the poc-0.2 patch I attached in my previous 
> > email.
> 
> Oh, you mean another value for the move_operation_enum enumeration?
> That's possible, but sounds too much, since all you need is another
> call to move_it_in_display_line.

Well, it is certainly a less invasive change that way so I'm going with it for now.
But I have to say it feels wasteful to call the function twice instead of having
it deal with it "correctly" the first time around with an option.

> 
> > > Dragging is handled by the same code: all we need is to know the
> > > beginning and the end buffer position.  I was more asking about user
> > > expectations: those could be different when clicking to move point and
> > > when dragging.
> > 
> > Well, from my personal user expectation: If there is an option to toggle this 
> > halfway behavior on, I would expect it to also toggle it on for dragging, 
> > unless there's a second option explicitly for dragging. Personally, I think 
> > one option to toggle both would be good.
> 
> Maybe.  We will need to hear more opinions, I guess.
> 

Sure, until then next up for me is figuring out where I'll need to make
mouse dragging behave in line with the "mouse-alternate-point" mode.

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diff --git a/src/dispnew.c b/src/dispnew.c
index 65d9cf9b4e1..bd288286924 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -22,6 +22,7 @@ Copyright (C) 1985-1988, 1993-1995, 1997-2023 Free Software Foundation,
 
 #include "sysstdio.h"
 #include <stdlib.h>
+#include <math.h>
 #include <unistd.h>
 
 #include "lisp.h"
@@ -5611,6 +5612,9 @@ buffer_posn_from_coords (struct window *w, int *x, int *y, struct display_pos *p
      argument is ZV to prevent move_it_in_display_line from matching
      based on buffer positions.  */
   move_it_in_display_line (&it, ZV, to_x, MOVE_TO_X);
+  if (mouse_point_alternate && to_x > it.current_x + ceil (it.pixel_width / 2.0))
+    move_it_in_display_line (&it, ZV, it.current_x + it.pixel_width, MOVE_TO_X);
+
   bidi_unshelve_cache (itdata, 0);
 
   Fset_buffer (old_current_buffer);
@@ -6468,6 +6472,7 @@ init_display_interactive (void)
 
   inverse_video = 0;
   cursor_in_echo_area = false;
+  mouse_point_alternate = false;
 
   /* Now is the time to initialize this; it's used by init_sys_modes
      during startup.  */
@@ -6788,6 +6793,11 @@ syms_of_display (void)
   DEFVAR_BOOL ("cursor-in-echo-area", cursor_in_echo_area,
 	       doc: /* Non-nil means put cursor in minibuffer, at end of any message there.  */);
 
+  DEFVAR_BOOL ("mouse-point-alternate", mouse_point_alternate,
+	       doc: /* Non-nil means use alternate mode for mouse click selection of point.
+In alternate mode, if the clicked x coordinate exceeds half of the selected
+display element's width, put point after it.  */);
+
   DEFVAR_LISP ("glyph-table", Vglyph_table,
 	       doc: /* Table defining how to output a glyph code to the frame.
 If not nil, this is a vector indexed by glyph code to define the glyph.