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

[Moritz Maxeiner <mm@ucw.sh>]

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Thank you for your suggestions (I've not quoted them in this reply for 
brevity). Unless I've overlooked something I've implemented them all, except 
for the etc/NEWS entry.

On Monday 10 July 2023 14:46:39 CEST Eli Zaretskii wrote:
> It should also be called out in etc/NEWS.

I agree, but I feel this should be done in the last iteration of the patch, 
once all other changes are done. Which leads me to the next point:

I had to add another piece of code to src/xterm.c, because while text 
selection via mouse dragging did automatically factor in the new optional 
behavior after releasing the mouse, it did not update the highlighted / 
selected region, as that requires mouse-movement events to be triggered, which 
are picked up in mouse.el via mouse-drag-track, which then updated the 
highlighted region. That event, however, is only triggered by leaving the 
previous glyph, so we need to trigger it while still being on the glyph.

I tried sending the event only when crossing the vertical midline of the 
glyph, but for a reason unknown to me, sending the event only once did not 
result in the highlight being updated when dragging the mouse to the right, 
only to the left.

As a workaround, I now trigger the event always while dragging with the 
optional behavior on. If there is a more elegant solution I don't see I'd 
welcome it. For now the overhead doesn't seem noticeable in practice.

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diff --git a/lisp/cus-start.el b/lisp/cus-start.el
index 054683d7cf6..489471f8eab 100644
--- a/lisp/cus-start.el
+++ b/lisp/cus-start.el
@@ -231,6 +231,7 @@ minibuffer-prompt-properties--setter
 	     (inverse-video display boolean)
 	     (visible-bell display boolean)
 	     (no-redraw-on-reenter display boolean)
+	     (mouse-click-prefer-closest-char display boolean)
 
              ;; doc.c
              (text-quoting-style display
diff --git a/src/dispnew.c b/src/dispnew.c
index 65d9cf9b4e1..5fcecd5811d 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_click_prefer_closest_char)
+    {
+      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 +6797,12 @@ 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-click-prefer-closest-char", mouse_click_prefer_closest_char,
+	       doc: /* Non-nil means mouse click prefers the closest glyph as point.
+When this is non-nil, clicking inside a glyph picks up the next glyph if the click
+is closer to it then half the width of the clicked glyph.  */);
+  mouse_click_prefer_closest_char = 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/xterm.c b/src/xterm.c
index 5840b15bcb7..44dcc8b1761 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -14206,11 +14206,13 @@ x_note_mouse_movement (struct frame *frame, const XMotionEvent *event,
     }
 
 
-  /* Has the mouse moved off the glyph it was on at the last sighting?  */
+  /* Has the mouse moved off the glyph it was on at the last sighting?
+     Is the mouse being dragged while we need to keep point to the nearest glyph? */
   r = &dpyinfo->last_mouse_glyph;
   if (frame != dpyinfo->last_mouse_glyph_frame
       || event->x < r->x || event->x >= r->x + r->width
-      || event->y < r->y || event->y >= r->y + r->height)
+      || event->y < r->y || event->y >= r->y + r->height
+      || mouse_click_prefer_closest_char && EQ (track_mouse, Qdrag_tracking))
     {
       frame->mouse_moved = true;
       frame->last_mouse_device = device;
@@ -31705,4 +31707,6 @@ syms_of_xterm (void)
 If that is still too slow, setting this variable to the symbol
 `really-fast' will make Emacs return only cached values.  */);
   Vx_use_fast_mouse_position = Qnil;
+
+  DEFSYM (Qdrag_tracking, "drag-tracking");
 }