Subject: [PATCH] Fix mouse highlighting for compact mode lines
From: Pengji Zhang <[email-masked]>
To: [email-masked]
Date: Fri, 28 Mar 2025 18:55:30 +0800
List-Id: <debbugs-submit.debbugs.gnu.org>

Hello,

I found that with 'mode-line-compact' being non-nil, mouse highlighting
would not work properly. To reproduce:

  - emacs -Q
  - (setq mode-line-compact t) or (setq mode-line-compact 'long)
  - Hover mouse cursor over the major mode part on the mode line

Now the minor mode lighters are also highlighted. That is presumably
because when 'mode-line-compact' is non-nil, the whole mode line string
is displayed at once. So the glyphs produced all have the same 'object'
field. That confuses the computation of boundaries for mouse
highlighting.

In this patch we instead split the mode line string into elements and
display them one by one.

Thanks!

From 5c7538aceb7e80ea44be7d74890d36e88d36169c Mon Sep 17 00:00:00 2001
From: Pengji Zhang <[email-masked]>
Date: Sun, 23 Mar 2025 20:19:09 +0800
Subject: [PATCH] Fix mouse highlighting for compact mode lines

When 'mode-line-compact' is non-nil, the mode line string is
displayed as a whole.  That confuses the computation of ranges
of mouse highlighting on the mode line because all the glyphs
have the same Lisp object source.  As such, in this commit we
instead split the mode line string by sources, and display those
elements one by one, so the boundaries of each element could be
correctly detected for the purpose of mouse highlighting.

* src/xdisp.c (display_mode_line): Display mode line elements
one by one when 'mode-line-compact' is non-nil.
(display_mode_element): Record source of the stored string via a
text property.
(syms_of_xdisp): New symbol for the text property.
---
 src/xdisp.c | 132 ++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 93 insertions(+), 39 deletions(-)

diff --git a/src/xdisp.c b/src/xdisp.c
index f2b158f00e3..193f9767c6f 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -27745,57 +27745,102 @@ display_mode_line (struct window *w, enum face_id face_id, Lisp_Object format)
 	{
 	  /* The window is wide enough; just display the mode line we
 	     just computed. */
-	  display_string (NULL, mode_string, Qnil,
-			  0, 0, &it, 0, 0, 0,
-			  STRING_MULTIBYTE (mode_string));
+	  Lisp_Object start = make_fixnum (0), end;
+	  Lisp_Object elt;
+	  AUTO_LIST2 (props, Qmode_line_element, Qnil);
+
+	  /* Display the mode line string one element by one element.
+	     This is to make the ranges of mouse highlighting
+	     correct. */
+	  do
+	    {
+	      end = Fnext_single_property_change (start, Qmode_line_element,
+						  mode_string, Qnil);
+	      elt = Fsubstring (mode_string, start, end);
+	      Fremove_text_properties (make_fixnum (0),
+				       make_fixnum (SCHARS (elt)),
+				       props, elt);
+	      display_string (NULL, elt, Qnil, 0, 0, &it, 0, 0, 0,
+			      STRING_MULTIBYTE (elt));
+	      start = end;
+	    }
+	  while (!NILP (end));
 	}
       else
 	{
 	  /* Compress the mode line. */
-	  ptrdiff_t i = 0, i_byte = 0, start = 0;
+	  ptrdiff_t i = 0, i_byte = 0;
 	  int prev = 0;
+	  Lisp_Object start = make_fixnum (0), end;
+	  Lisp_Object elt = empty_unibyte_string;
+	  AUTO_LIST2 (props, Qmode_line_element, Qnil);
 
-	  while (i < SCHARS (mode_string))
+	  /* Display the mode line string one element by one element.
+	     This is to make the ranges of mouse highlighting
+	     correct. */
+	  do
 	    {
-	      int c = fetch_string_char_advance (mode_string, &i, &i_byte);
-	      if (c == ' ' && prev == ' ')
+	      end = Fnext_single_property_change (start,
+						  Qmode_line_element,
+						  mode_string,
+						  make_fixnum (SCHARS (mode_string)));
+	      while (i < XFIXNUM (end))
 		{
-		  Lisp_Object prev_pos = make_fixnum (i - 1);
-
-		  /* SPC characters with 'display' properties are not
-                     really "empty", since they have non-trivial visual
-                     effects on the mode line.  */
-		  if (NILP (Fget_text_property (prev_pos, Qdisplay,
-						mode_string)))
+		  int c = fetch_string_char_advance (mode_string, &i, &i_byte);
+		  if (c == ' ' && prev == ' ')
 		    {
-		      display_string (NULL,
-				      Fsubstring (mode_string,
-						  make_fixnum (start),
-						  prev_pos),
-				      Qnil, 0, 0, &it, 0, 0, 0,
-				      STRING_MULTIBYTE (mode_string));
-		      /* Skip past the rest of the space characters. */
-		      while (c == ' ' && i < SCHARS (mode_string)
-			     && NILP (Fget_text_property (make_fixnum (i),
-							  Qdisplay,
-							  mode_string)))
+		      Lisp_Object prev_pos = make_fixnum (i - 1);
+		      Lisp_Object display = Fget_text_property (prev_pos,
+								Qdisplay,
+								mode_string);
+
+		      /* SPC characters with 'display' properties are not
+			 really "empty", since they have non-trivial visual
+			 effects on the mode line.  */
+		      if (NILP (display))
 			{
-			  c = fetch_string_char_advance (mode_string,
-							 &i, &i_byte);
+			  elt = concat2 (elt, Fsubstring (mode_string,
+							  start,
+							  prev_pos));
+
+			  /* Skip past the rest of the space characters. */
+			  Lisp_Object display = Fget_text_property (make_fixnum (i),
+								    Qdisplay,
+								    mode_string);
+			  while (c == ' ' && i < XFIXNUM (end)
+				 && NILP (display))
+			    {
+			      c = fetch_string_char_advance (mode_string,
+							     &i, &i_byte);
+			      display = Fget_text_property (make_fixnum (i),
+							    Qdisplay,
+							    mode_string);
+			    }
+
+			  /* Skip the final space no matter how the loop
+			     above ends. */
+			  if (c == ' ' && NILP (display))
+			    start = end;
+			  else
+			    start = make_fixnum (i - 1);
 			}
-		      start = i - 1;
 		    }
+		  prev = c;
 		}
-	      prev = c;
-	    }
 
-	  /* Display the final bit. */
-	  if (start < i)
-	    display_string (NULL,
-			    Fsubstring (mode_string, make_fixnum (start),
-					make_fixnum (i)),
-			    Qnil, 0, 0, &it, 0, 0, 0,
-			    STRING_MULTIBYTE (mode_string));
+	      /* Append the final bit. */
+	      if (XFIXNUM (start) < XFIXNUM (end))
+		elt = concat2 (elt, Fsubstring (mode_string, start, end));
+
+	      Fremove_text_properties (make_fixnum (0),
+				       make_fixnum (SCHARS (elt)),
+				       props, elt);
+	      display_string (NULL, elt, Qnil, 0, 0, &it, 0, 0, 0,
+			      STRING_MULTIBYTE (elt));
+	      elt = empty_unibyte_string;
+	      start = end;
+	    }
+	  while (XFIXNUM (end) < SCHARS (mode_string));
 	}
     }
   pop_kboard ();
@@ -28014,7 +28059,10 @@ display_mode_element (struct it *it, int depth, int field_width, int precision,
 		n += store_mode_line_noprop (SSDATA (elt), -1, prec);
 		break;
 	      case MODE_LINE_STRING:
-		n += store_mode_line_string (NULL, elt, true, 0, prec, Qnil);
+		{
+		  AUTO_LIST2 (src, Qmode_line_element, elt);
+		  n += store_mode_line_string (NULL, elt, true, 0, prec, src);
+		}
 		break;
 	      case MODE_LINE_DISPLAY:
 		n += display_string (NULL, elt, Qnil, 0, 0, it,
@@ -28067,8 +28115,9 @@ display_mode_element (struct it *it, int depth, int field_width, int precision,
 		      Lisp_Object mode_string
 			= Fsubstring (elt, make_fixnum (charpos),
 				      make_fixnum (endpos));
+		      AUTO_LIST2 (src, Qmode_line_element, elt);
 		      n += store_mode_line_string (NULL, mode_string, false,
-						   0, 0, Qnil);
+						   0, 0, src);
 		    }
 		    break;
 		  case MODE_LINE_DISPLAY:
@@ -28148,6 +28197,7 @@ display_mode_element (struct it *it, int depth, int field_width, int precision,
 			{
 			  Lisp_Object tem = build_string (spec);
 			  props = Ftext_properties_at (make_fixnum (charpos), elt);
+			  props = plist_put (props, Qmode_line_element, elt);
 			  /* Should only keep face property in props */
 			  n += store_mode_line_string (NULL, tem, false,
 						       field, prec, props);
@@ -28363,6 +28413,9 @@ display_mode_element (struct it *it, int depth, int field_width, int precision,
 	  n += store_mode_line_noprop ("", field_width - n, 0);
 	  break;
 	case MODE_LINE_STRING:
+	  /* NOTE: Padding implicitly has 'mode-line-element' property
+	     to be nil.  Normally padding spaces are between two real
+	     elements, so this should be good. */
 	  n += store_mode_line_string ("", Qnil, false, field_width - n, 0,
 				       Qnil);
 	  break;
@@ -37657,6 +37710,7 @@ syms_of_xdisp (void)
   DEFSYM (Qfontification_functions, "fontification-functions");
   DEFSYM (Qlong_line_optimizations_in_fontification_functions,
 	  "long-line-optimizations-in-fontification-functions");
+  DEFSYM (Qmode_line_element, "mode-line-element");
 
   /* Name of the symbol which disables Lisp evaluation in 'display'
      properties.  This is used by enriched.el.  */
-- 
2.49.0