fix(column): use a single path for sign sorting (#27431)

Problem:  #25826 added a (duplicate) sign comparison function, which was
modified and strayed from the original in #27418.
Solution: Merge the two functions and add a display test that actually
tests for this order in addition to the legacy tests.

src/nvim/decoration.c

@@ -664,11 +664,6 @@ next_mark:
   return attr;
 }
 
-typedef struct {
-  DecorSignHighlight *sh;
-  uint32_t id;
-} SignItem;
-
 int sign_item_cmp(const void *p1, const void *p2)
 {
   const SignItem *s1 = (SignItem *)p1;
@@ -683,7 +678,7 @@ int sign_item_cmp(const void *p1, const void *p2)
   }
 
   if (s1->sh->sign_add_id != s2->sh->sign_add_id) {
-    return s1->sh->sign_add_id > s2->sh->sign_add_id ? 1 : -1;
+    return s1->sh->sign_add_id < s2->sh->sign_add_id ? 1 : -1;
   }
 
   return 0;

src/nvim/sign.c

@@ -168,7 +168,7 @@ static int buf_findsign(buf_T *buf, int id, char *group)
 }
 
 /// qsort() function to sort signs by line number, priority, id and recency.
-int sign_cmp(const void *p1, const void *p2)
+static int sign_row_cmp(const void *p1, const void *p2)
 {
   const MTKey *s1 = (MTKey *)p1;
   const MTKey *s2 = (MTKey *)p2;
@@ -180,20 +180,10 @@ int sign_cmp(const void *p1, const void *p2)
   DecorSignHighlight *sh1 = decor_find_sign(mt_decor(*s1));
   DecorSignHighlight *sh2 = decor_find_sign(mt_decor(*s2));
   assert(sh1 && sh2);
+  SignItem si1 = { sh1, s1->id };
+  SignItem si2 = { sh2, s2->id };
 
-  if (sh1->priority != sh2->priority) {
-    return sh1->priority < sh2->priority ? 1 : -1;
-  }
-
-  if (s1->id != s2->id) {
-    return s1->id < s2->id ? 1 : -1;
-  }
-
-  if (sh1->sign_add_id != sh2->sign_add_id) {
-    return sh1->sign_add_id < sh2->sign_add_id ? 1 : -1;
-  }
-
-  return 0;
+  return sign_item_cmp(&si1, &si2);
 }
 
 /// Delete the specified sign(s)
@@ -249,7 +239,7 @@ static int buf_delete_signs(buf_T *buf, char *group, int id, linenr_T atlnum)
 
   // Sort to remove the highest priority sign at a specific line number.
   if (kv_size(signs)) {
-    qsort((void *)&kv_A(signs, 0), kv_size(signs), sizeof(MTKey), sign_cmp);
+    qsort((void *)&kv_A(signs, 0), kv_size(signs), sizeof(MTKey), sign_row_cmp);
     extmark_del_id(buf, kv_A(signs, 0).ns, kv_A(signs, 0).id);
     kv_destroy(signs);
   } else if (atlnum > 0) {
@@ -304,7 +294,7 @@ static void sign_list_placed(buf_T *rbuf, char *group)
       }
 
       if (kv_size(signs)) {
-        qsort((void *)&kv_A(signs, 0), kv_size(signs), sizeof(MTKey), sign_cmp);
+        qsort((void *)&kv_A(signs, 0), kv_size(signs), sizeof(MTKey), sign_row_cmp);
 
         for (size_t i = 0; i < kv_size(signs); i++) {
           namebuf[0] = '\0';
@@ -1002,7 +992,7 @@ static void sign_get_placed_in_buf(buf_T *buf, linenr_T lnum, int sign_id, const
   }
 
   if (kv_size(signs)) {
-    qsort((void *)&kv_A(signs, 0), kv_size(signs), sizeof(MTKey), sign_cmp);
+    qsort((void *)&kv_A(signs, 0), kv_size(signs), sizeof(MTKey), sign_row_cmp);
     for (size_t i = 0; i < kv_size(signs); i++) {
       tv_list_append_dict(l, sign_get_placed_info_dict(kv_A(signs, i)));
     }

src/nvim/sign_defs.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include "nvim/decoration_defs.h"
 #include "nvim/types_defs.h"
 
 /// Sign attributes. Used by the screen refresh routines.
@@ -19,5 +20,10 @@ typedef struct {
   int sn_num_hl;   // highlight ID for line number
 } sign_T;
 
+typedef struct {
+  DecorSignHighlight *sh;
+  uint32_t id;
+} SignItem;
+
 enum { SIGN_SHOW_MAX = 9, };  ///< Maximum number of signs shown on a single line
 enum { SIGN_DEF_PRIO = 10, };  ///< Default sign highlight priority

test/functional/ui/decorations_spec.lua

@@ -5160,6 +5160,18 @@ l5
                           |
     ]]}
   end)
+
+  it('correct sort order with multiple namespaces and same id', function()
+    local ns2 = api.nvim_create_namespace('')
+    api.nvim_buf_set_extmark(0, ns, 0, 0, {sign_text = 'S1', id = 1})
+    api.nvim_buf_set_extmark(0, ns2, 0, 0, {sign_text = 'S2', id = 1})
+
+    screen:expect{grid=[[
+      S1S2^                                              |
+      {2:~                                                 }|*8
+                                                        |
+    ]]}
+  end)
 end)
 
 describe('decorations: virt_text', function()