fix(completion): avoid deleting text when completion leader changes #31448

Problem:  When completion leader changes, text that might be reinserted
          immediately after is deleted. This unnecessarily affects
          extmarks. #31387 restored the original extmarks but that
          prevents end_right_gravity marks from growing.
Solution: Avoid deleting leader text that will be reinserted.

src/nvim/edit.c

@@ -604,7 +604,7 @@ static int insert_execute(VimState *state, int key)
            || (ins_compl_enter_selects()
                && (s->c == CAR || s->c == K_KENTER || s->c == NL)))
           && stop_arrow() == OK) {
-        ins_compl_delete();
+        ins_compl_delete(false);
         ins_compl_insert(false);
       }
     }

src/nvim/insexpand.c

@@ -951,14 +951,14 @@ static void ins_compl_longest_match(compl_T *match)
     compl_leader = xstrdup(match->cp_str);
 
     bool had_match = (curwin->w_cursor.col > compl_col);
-    ins_compl_delete();
+    ins_compl_delete(false);
     ins_bytes(compl_leader + get_compl_len());
     ins_redraw(false);
 
     // When the match isn't there (to avoid matching itself) remove it
     // again after redrawing.
     if (!had_match) {
-      ins_compl_delete();
+      ins_compl_delete(false);
     }
     compl_used_match = false;
 
@@ -985,14 +985,14 @@ static void ins_compl_longest_match(compl_T *match)
     // Leader was shortened, need to change the inserted text.
     *p = NUL;
     bool had_match = (curwin->w_cursor.col > compl_col);
-    ins_compl_delete();
+    ins_compl_delete(false);
     ins_bytes(compl_leader + get_compl_len());
     ins_redraw(false);
 
     // When the match isn't there (to avoid matching itself) remove it
     // again after redrawing.
     if (!had_match) {
-      ins_compl_delete();
+      ins_compl_delete(false);
     }
   }
 
@@ -1811,9 +1811,8 @@ static bool ins_compl_need_restart(void)
 static void ins_compl_new_leader(void)
 {
   ins_compl_del_pum();
-  ins_compl_delete();
+  ins_compl_delete(true);
   ins_bytes(compl_leader + get_compl_len());
-  restore_orig_extmarks();
   compl_used_match = false;
 
   if (compl_started) {
@@ -2137,7 +2136,7 @@ static bool ins_compl_stop(const int c, const int prev_mode, bool retval)
   // CTRL-E means completion is Ended, go back to the typed text.
   // but only do this, if the Popup is still visible
   if (c == Ctrl_E) {
-    ins_compl_delete();
+    ins_compl_delete(false);
     char *p = NULL;
     if (compl_leader != NULL) {
       p = compl_leader;
@@ -3610,11 +3609,24 @@ static void ins_compl_update_shown_match(void)
 }
 
 /// Delete the old text being completed.
-void ins_compl_delete(void)
+void ins_compl_delete(bool new_leader)
 {
+  // Avoid deleting text that will be reinserted when changing leader. This
+  // allows marks present on the original text to shrink/grow appropriately.
+  int orig_col = 0;
+  if (new_leader) {
+    char *orig = compl_orig_text;
+    char *leader = ins_compl_leader();
+    while (*orig != NUL && utf_ptr2char(orig) == utf_ptr2char(leader)) {
+      leader += utf_ptr2len(leader);
+      orig += utf_ptr2len(orig);
+    }
+    orig_col = (int)(orig - compl_orig_text);
+  }
+
   // In insert mode: Delete the typed part.
   // In replace mode: Put the old characters back, if any.
-  int col = compl_col + (compl_status_adding() ? compl_length : 0);
+  int col = compl_col + (compl_status_adding() ? compl_length : orig_col);
   if ((int)curwin->w_cursor.col > col) {
     if (stop_arrow() == FAIL) {
       return;
@@ -3858,7 +3870,7 @@ static int ins_compl_next(bool allow_get_expansion, int count, bool insert_match
   if (allow_get_expansion && insert_match
       && (!compl_get_longest || compl_used_match)) {
     // Delete old text to be replaced
-    ins_compl_delete();
+    ins_compl_delete(false);
   }
 
   // When finding the longest common text we stick at the original text,
@@ -3911,7 +3923,7 @@ static int ins_compl_next(bool allow_get_expansion, int count, bool insert_match
 
     // Delete old text to be replaced, since we're still searching and
     // don't want to match ourselves!
-    ins_compl_delete();
+    ins_compl_delete(false);
   }
 
   // Enter will select a match when the match wasn't inserted and the popup

test/functional/editor/completion_spec.lua

@@ -1265,7 +1265,7 @@ describe('completion', function()
     command([[
       call setline(1, ['aaaa'])
       let ns_id = nvim_create_namespace('extmark')
-      let mark_id = nvim_buf_set_extmark(0, ns_id, 0, 0, { 'end_col':2, 'hl_group':'Error'})
+      let mark_id = nvim_buf_set_extmark(0, ns_id, 0, 0, { 'end_col':2, 'hl_group':'Error' })
       let mark = nvim_buf_get_extmark_by_id(0, ns_id, mark_id, { 'details':1 })
       inoremap <C-x> <C-r>=Complete()<CR>
       function Complete() abort
@@ -1312,5 +1312,18 @@ describe('completion', function()
         {5:-- Keyword completion (^N^P) }{19:Back at original}               |
       ]],
     })
+    -- But still grows with end_right_gravity #31437
+    command(
+      "call nvim_buf_set_extmark(0, ns_id, 1, 0, { 'end_col':2, 'hl_group':'Error', 'end_right_gravity': 1 })"
+    )
+    feed('<Esc>ji<C-N>a')
+    screen:expect({
+      grid = [[
+        {9:aa}aaa                                                       |
+        {9:aaa}^aa                                                       |
+        aaaaa                                                       |
+        {5:-- INSERT --}                                                |
+      ]],
+    })
   end)
 end)