Implement new option 'mouse-prefer-closest-glyph'

* src/dispnew.c (mouse_prefer_closest_glyph): New global
variable.
(buffer_posn_from_coords):
* src/xdisp.c (remember_mouse_glyph): Respect
'mouse_prefer_closest_glyph'.
(mouse_fine_grained_tracking): Update documentation to
include 'mouse_prefer_closest_glyph' effects.

* doc/lispref/commands.texi (Accessing Mouse): Update
documentation to say what the new option does when enabled.
* lisp/cus-start.el (standard): New user option
'mouse-prefer-closest-glyph'.

doc/lispref/commands.texi

@@ -2756,6 +2756,16 @@ 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

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

src/dispnew.c

@@ -5636,6 +5636,15 @@ 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);
@@ -6813,6 +6822,15 @@ 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.

src/xdisp.c

@@ -2759,6 +2759,7 @@ 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)
     {
@@ -2769,6 +2770,8 @@ 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;
@@ -2984,6 +2987,15 @@ 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.  */
@@ -37503,7 +37515,10 @@ 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,