Optionally allow point to enter composed character sequences

This changeset is based on code originally posted by
Kenichi Handa <handa@gnu.org> as part of discussing bug#20140,
with a few adjustments and changes, mainly to support R2L
text and provide a user option to control the feature.
* src/xdisp.c (compute_stop_pos, next_element_from_buffer)
(composition_reseat_it): Don't allow auto-composing characters
past point when 'composition-break-at-point' is non-nil.
(syms_of_xdisp) <composition-break-at-point>: New boolean variable.
(redisplay_internal, redisplay_window): Disable "just move the
cursor" optimizations when 'composition-break-at-point' is non-nil.
* src/keyboard.c (command_loop_1): Force redisplay when
'composition-break-at-point' is non-nil.

* lisp/cus-start.el (standard): Provide customization form for
'composition-break-at-point'.

* etc/NEWS: Announce the new feature.

5 files changed, 79 insertions, 19 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 6f19224026e..1b0e26da9b9 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -476,6 +476,17 @@ The options 'mouse-wheel-down-alternate-event', 'mouse-wheel-up-alternate-event'
 been added to better support systems where two kinds of wheel events can be
 received.
 
+** Editing complex text layout (CTL) scripts
+
+*** The <Delete> function key now allows deleting the entire composed sequence.
+For the details, see the item about the 'delete-forward-char' command
+above.
+
+*** New user option 'composition-break-at-point'.
+Setting it to a non-nil value temporarily disables automatic
+composition of character sequences at point, and thus makes it easier
+to edit such sequences by allowing point to "enter" the sequence.
+
 
 * Changes in Specialized Modes and Packages in Emacs 29.1
 
diff --git a/lisp/cus-start.el b/lisp/cus-start.el
index afdbd82457b..f8bab89e902 100644
--- a/lisp/cus-start.el
+++ b/lisp/cus-start.el
@@ -811,6 +811,7 @@ since it could result in memory overflow and make Emacs crash."
                character)
               "27.1"
               :safe (lambda (value) (or (characterp value) (null value))))
+             (composition-break-at-point display boolean "29.1")
 	     ;; xfaces.c
 	     (scalable-fonts-allowed
               display (choice (const :tag "Don't allow scalable fonts" nil)
diff --git a/src/composite.c b/src/composite.c
index 17563201e27..c3e9afc8078 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -1292,6 +1292,16 @@ composition_reseat_it (struct composition_it *cmp_it, ptrdiff_t charpos,
 	  if (cmp_it->lookback > 0)
 	    {
 	      cpos = charpos - cmp_it->lookback;
+	      /* Reject the composition if it starts before ENDPOS,
+		 which here can only happen if
+		 composition-break-at-point is non-nil and point is
+		 inside the composition.  */
+	      if (cpos < endpos)
+		{
+		  eassert (composition_break_at_point);
+		  eassert (endpos == PT);
+		  goto no_composition;
+		}
 	      if (STRINGP (string))
 		bpos = string_char_to_byte (string, cpos);
 	      else
diff --git a/src/keyboard.c b/src/keyboard.c
index 6b66a715933..be9fb665d76 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -1603,23 +1603,33 @@ command_loop_1 (void)
 
       if (current_buffer == prev_buffer
 	  && XBUFFER (XWINDOW (selected_window)->contents) == current_buffer
-	  && last_point_position != PT
-	  && NILP (Vdisable_point_adjustment)
-	  && NILP (Vglobal_disable_point_adjustment))
+	  && last_point_position != PT)
 	{
-	  if (last_point_position > BEGV
-	      && last_point_position < ZV
-	      && (composition_adjust_point (last_point_position,
-					    last_point_position)
-		  != last_point_position))
-	    /* The last point was temporarily set within a grapheme
-	       cluster to prevent automatic composition.  To recover
-	       the automatic composition, we must update the
-	       display.  */
-	    windows_or_buffers_changed = 21;
-	  if (!already_adjusted)
-	    adjust_point_for_property (last_point_position,
-				       MODIFF != prev_modiff);
+	  if (NILP (Vdisable_point_adjustment)
+	      && NILP (Vglobal_disable_point_adjustment)
+	      && !composition_break_at_point)
+	    {
+	      if (last_point_position > BEGV
+		  && last_point_position < ZV
+		  && (composition_adjust_point (last_point_position,
+						last_point_position)
+		      != last_point_position))
+		/* The last point was temporarily set within a grapheme
+		   cluster to prevent automatic composition.  To recover
+		   the automatic composition, we must update the
+		   display.  */
+		windows_or_buffers_changed = 21;
+	      if (!already_adjusted)
+		adjust_point_for_property (last_point_position,
+					   MODIFF != prev_modiff);
+	    }
+	  else if (PT > BEGV && PT < ZV
+		   && (composition_adjust_point (last_point_position, PT)
+		       != PT))
+	    /* Now point is within a grapheme cluster.  We must update
+	       the display so that this cluster is de-composed on the
+	       screen and the cursor is correctly placed at point.  */
+	    windows_or_buffers_changed = 39;
 	}
 
       /* Install chars successfully executed in kbd macro.  */
diff --git a/src/xdisp.c b/src/xdisp.c
index cc1da24acaf..ec8d53e5ccb 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3984,6 +3984,12 @@ compute_stop_pos (struct it *it)
       pos = next_overlay_change (charpos);
       if (pos < it->stop_charpos)
 	it->stop_charpos = pos;
+      /* If we are breaking compositions at point, stop at point.  */
+      if (!NILP (BVAR (current_buffer, enable_multibyte_characters))
+	  && !NILP (Vauto_composition_mode)
+	  && composition_break_at_point
+	  && charpos < PT && PT < it->stop_charpos)
+	it->stop_charpos = PT;
 
       /* Set up variables for computing the stop position from text
          property changes.  */
@@ -3995,7 +4001,8 @@ compute_stop_pos (struct it *it)
 	 chunks.  We play safe here by assuming that only SPC, TAB,
 	 FF, and NL cannot be in some composition; in particular, most
 	 ASCII punctuation characters could be composed into ligatures.  */
-      if (!NILP (BVAR (current_buffer, enable_multibyte_characters))
+      if (!composition_break_at_point
+	  && !NILP (BVAR (current_buffer, enable_multibyte_characters))
 	  && !NILP (Vauto_composition_mode))
 	{
 	  ptrdiff_t endpos = charpos + 10 * TEXT_PROP_DISTANCE_LIMIT;
@@ -9186,7 +9193,19 @@ next_element_from_buffer (struct it *it)
 	  && IT_CHARPOS (*it) >= it->redisplay_end_trigger_charpos)
 	run_redisplay_end_trigger_hook (it);
 
-      stop = it->bidi_it.scan_dir < 0 ? -1 : it->end_charpos;
+      if (composition_break_at_point
+	  && !NILP (BVAR (current_buffer, enable_multibyte_characters))
+	  && !NILP (Vauto_composition_mode))
+	{
+	  /* Limit search for composable characters to point's position.  */
+	  if (it->bidi_it.scan_dir < 0)
+	    stop = (PT <= IT_CHARPOS (*it)) ? PT : -1;
+	  else
+	    stop = (IT_CHARPOS (*it) < PT
+		    && PT < it->end_charpos) ? PT : it->end_charpos;
+	}
+      else
+	stop = it->bidi_it.scan_dir < 0 ? -1 : it->end_charpos;
       if (CHAR_COMPOSED_P (it, IT_CHARPOS (*it), IT_BYTEPOS (*it),
 			   stop)
 	  && next_element_from_composition (it))
@@ -16390,7 +16409,8 @@ redisplay_internal (void)
       /* If highlighting the region, or if the cursor is in the echo area,
 	 then we can't just move the cursor.  */
       else if (NILP (Vshow_trailing_whitespace)
-	       && !cursor_in_echo_area)
+	       && !cursor_in_echo_area
+	       && !composition_break_at_point)
 	{
 	  struct it it;
 	  struct glyph_row *row;
@@ -18928,6 +18948,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
        && !current_buffer->clip_changed
        && !current_buffer->prevent_redisplay_optimizations_p
        && !window_outdated (w)
+       && !composition_break_at_point
        && !hscrolling_current_line_p (w));
 
   beg_unchanged = BEG_UNCHANGED;
@@ -36527,6 +36548,13 @@ Otherwise, use custom-tailored code after resizing minibuffer windows to try
 and display the most important part of the minibuffer.   */);
   /* See bug#43519 for some discussion around this.  */
   redisplay_adhoc_scroll_in_resize_mini_windows = true;
+
+  DEFVAR_BOOL ("composition-break-at-point",
+	       composition_break_at_point,
+	       doc: /* If non-nil, prevent auto-composition of characters around point.
+This makes it easier to edit character sequences that are
+composed on display.  */);
+  composition_break_at_point = false;
 }