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

[Moritz Maxeiner <mm@ucw.sh>]

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On Thursday 13 July 2023 10:47:36 CEST Eli Zaretskii wrote:
> > From: Moritz Maxeiner <mm@ucw.sh>
> > Cc: emacs-devel@gnu.org
> > Date: Wed, 12 Jul 2023 21:58:44 +0200
> > 
> > +@defvar mouse-prefer-closest-glyph
> 
> This is a user option, so please use @defopt, not @defvar.
> 
> Also, please use some smaller (preferably, default) value for
> fill-column, as the text you posted uses too long lines.
> 
> > +If you set this variable to non-@code{nil}, whenever you click or drag
> > the mouse, +instead of the point being always set in front of the clicked
> > glyph, the point +horizontally closest to the mouse position will be
> > used.
> > +So if you click in the left half of a glyph, point is set in front of it,
> > +but if you click in the right half, point is set after it.
> 
> There's too much of passive voice here.  I think the alternative
> wording proposed by Po Lu is better.
> 
> > ++++
> > +** New user option 'mouse-prefer-closest-glyph'.
> > +When enabled, clicking or dragging with the mouse will put the point
> > +in front of the glyph with the closest x coordinate to the mouse pointer.
> 
>                                                        ^^^^^^^^^^^^^^^^^^^^
> "to the click or start of the drag"
> 
> > +In other words, if the mouse pointer is in the right half of a glyph,
> > +point will be put after that glyph, while if the mouse pointer is in the
> > +left half of a glyph, point will be put in front of that glyph.
> 
> Point is buffer position, whereas "glyph" is something on display.  So
> this should say something like "point will be put after the buffer
> position corresponding to that glyph".
> 
> Also, please state explicitly that the default of this feature is OFF.
> 
> I understand that the details of the code are still under discussion,
> but I thought I'd post these comments anyway.
> 
> Thanks.

Thank you both for your continued feedback. I haven't had time to work on this 
in a bit, but I think the patch now includes all your latest suggestions 
(sorry if I overlooked something).

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diff --git a/doc/lispref/commands.texi b/doc/lispref/commands.texi
index cd1745614eb..d1b8f9c494c 100644
--- a/doc/lispref/commands.texi
+++ b/doc/lispref/commands.texi
@@ -2751,6 +2751,16 @@ If @var{whole} is non-@code{nil}, the @var{x} coordinate is relative
 to the entire window area including scroll bars, margins and fringes.
 @end defun
 
+@defopt mouse-prefer-closest-glyph
+If this variable is non-@code{nil}, the @code{posn-point} of a mouse
+position list will be set to the position of the glyph whose left most
+position is closest to the mouse pointer, as opposed to the position of
+the glyph underneath the mouse pointer itself.  For example, if
+@code{posn-at-x-y} is called with @var{x} set to @code{9}, which is
+contained within a character of width 10 positioned at column 0, the
+point saved within the mouse position list will be after that character.
+@end defopt
+
 @node Accessing Scroll
 @subsection Accessing Scroll Bar Events
 @cindex scroll bar events, data in
diff --git a/etc/NEWS b/etc/NEWS
index 9e6f0c16bcd..348152cb7be 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -952,6 +952,17 @@ wheel reports.  Unlike 'pixel-scroll-mode', this mode scrolls the
 display pixel-by-pixel, as opposed to only animating line-by-line
 scrolls.
 
++++
+** New user option 'mouse-prefer-closest-glyph'.
+When enabled, clicking or dragging with the mouse will put the point
+in front of the buffer position corresponding to the glyph with the
+closest x coordinate to the click or start of the drag.
+In other words, if the mouse pointer is in the right half of a glyph,
+point will be put after the buffer position corresponding to that glyph,
+whereas if the mouse pointer is in the left half of a glyph, point
+will be put in front the buffer position corresponding to that glyph.
+This new option defaults to off.
+
 ** Terminal Emacs
 
 ---
diff --git a/lisp/cus-start.el b/lisp/cus-start.el
index 054683d7cf6..2ed904f178f 100644
--- a/lisp/cus-start.el
+++ b/lisp/cus-start.el
@@ -231,6 +231,7 @@ Leaving \"Default\" unchecked is equivalent with specifying a default of
 	     (inverse-video display boolean)
 	     (visible-bell display boolean)
 	     (no-redraw-on-reenter display boolean)
+	     (mouse-prefer-closest-glyph display boolean)
 
              ;; doc.c
              (text-quoting-style display
diff --git a/src/dispnew.c b/src/dispnew.c
index 65d9cf9b4e1..143dda412e9 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -5611,6 +5611,15 @@ 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_prefer_closest_glyph)
+    {
+      int next_x = it.current_x + it.pixel_width;
+      int before_dx = to_x - it.current_x;
+      int after_dx = next_x - to_x;
+      if (before_dx > after_dx)
+        move_it_in_display_line (&it, ZV, next_x, MOVE_TO_X);
+    }
+
   bidi_unshelve_cache (itdata, 0);
 
   Fset_buffer (old_current_buffer);
@@ -6788,6 +6797,15 @@ predicates which report frame's specific UI-related capabilities.  */);
   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-prefer-closest-glyph", mouse_prefer_closest_glyph,
+	       doc: /* Non-nil means mouse position lists are reported relative
+to the glyph closest to their coordinates.
+
+ When non-nil, mouse position lists will be reported with their
+`posn-point' set to the position of the glyph closest to the mouse
+pointer, instead of the glyph immediately under.  */);
+  mouse_prefer_closest_glyph = false;
+
   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.
diff --git a/src/xdisp.c b/src/xdisp.c
index 763af7d3bc8..1d71bb02641 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -2723,6 +2723,7 @@ remember_mouse_glyph (struct frame *f, int gx, int gy, NativeRectangle *rect)
   enum window_part part;
   enum glyph_row_area area;
   int x, y, width, height;
+  int original_gx;
 
   if (mouse_fine_grained_tracking)
     {
@@ -2733,6 +2734,8 @@ remember_mouse_glyph (struct frame *f, int gx, int gy, NativeRectangle *rect)
   /* Try to determine frame pixel position and size of the glyph under
      frame pixel coordinates X/Y on frame F.  */
 
+  original_gx = gx;
+
   if (window_resize_pixelwise)
     {
       width = height = 1;
@@ -2948,6 +2951,15 @@ remember_mouse_glyph (struct frame *f, int gx, int gy, NativeRectangle *rect)
   gy += WINDOW_TOP_EDGE_Y (w);
 
  store_rect:
+  if (mouse_prefer_closest_glyph)
+    {
+      int half_width = width / 2;
+      width = half_width;
+
+      int bisection = gx + half_width;
+      if (original_gx > bisection)
+        gx = bisection;
+    }
   STORE_NATIVE_RECT (*rect, gx, gy, width, height);
 
   /* Visible feedback for debugging.  */
@@ -37345,7 +37357,10 @@ may be more familiar to users.  */);
   DEFVAR_BOOL ("mouse-fine-grained-tracking", mouse_fine_grained_tracking,
     doc: /* Non-nil for pixel-wise mouse-movement.
 When nil, mouse-movement events will not be generated as long as the
-mouse stays within the extent of a single glyph (except for images).  */);
+mouse stays within the extent of a single glyph (except for images).
+When nil and mouse-prefer-closest-glyph is non-nil, mouse-movement
+events will instead not be generated as long as the mouse stays within
+the extent of a single left/right half glyph (except for images).  */);
   mouse_fine_grained_tracking = false;
 
   DEFVAR_BOOL ("tab-bar--dragging-in-progress", tab_bar__dragging_in_progress,