diff --git a/src/insexpand.c b/src/insexpand.c
index c231aa41b8..38b2095632 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -7690,17 +7690,26 @@ remove_old_matches(void)
     static void
 get_cpt_func_completion_matches(callback_T *cb UNUSED)
 {
-    int	startcol = cpt_sources_array[cpt_sources_index].cs_startcol;
+    cpt_source_T    *cpt_src = &cpt_sources_array[cpt_sources_index];
+    int		    startcol = cpt_src->cs_startcol;
 
     if (startcol == -2 || startcol == -3)
 	return;
 
     if (set_compl_globals(startcol, curwin->w_cursor.col, TRUE) == OK)
     {
+	// Insert the leader string (previously removed) before expansion.
+	// This prevents flicker when `func` (e.g. an LSP client) is slow and
+	// calls 'sleep', which triggers out_flush().
+	if (!cpt_src->cs_refresh_always)
+	    ins_compl_insert_bytes(ins_compl_leader(), -1);
+
 	expand_by_function(0, cpt_compl_pattern.string, cb);
 
-	cpt_sources_array[cpt_sources_index].cs_refresh_always =
-	    compl_opt_refresh_always;
+	if (!cpt_src->cs_refresh_always)
+	    ins_compl_delete();
+
+	cpt_src->cs_refresh_always = compl_opt_refresh_always;
 	compl_opt_refresh_always = FALSE;
     }
 }
diff --git a/src/testdir/test_ins_complete.vim b/src/testdir/test_ins_complete.vim
index b10e07433a..c2465030dd 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -586,15 +586,19 @@ func Test_completefunc_info()
   set completefunc&
 endfunc
 
-func Test_cpt_func_cursorcol()
+" For ^N completion, `completefunc` receives the same leader string in both the
+" 'info' and 'expansion' phases (the leader is not removed before expansion).
+" This avoids flicker when `completefunc` (e.g. an LSP client) is slow and calls
+" 'sleep', which triggers out_flush().
+func Test_completefunc_leader()
   func CptColTest(findstart, query)
     if a:findstart
-      call assert_equal(b:info_compl_line, getline(1))
-      call assert_equal(b:info_cursor_col, col('.'))
+      call assert_equal(b:compl_line, getline(1))
+      call assert_equal(b:cursor_col, col('.'))
       return col('.')
     endif
-    call assert_equal(b:expn_compl_line, getline(1))
-    call assert_equal(b:expn_cursor_col, col('.'))
+    call assert_equal(b:compl_line, getline(1))
+    call assert_equal(b:cursor_col, col('.'))
     return v:none
   endfunc
 
@@ -602,17 +606,13 @@ func Test_cpt_func_cursorcol()
   new
 
   " Replace mode
-  let b:info_compl_line = "foo barxyz"
-  let b:expn_compl_line = "foo barbaz"
-  let b:info_cursor_col = 10
-  let b:expn_cursor_col = 5
+  let b:compl_line = "foo barxyz"
+  let b:cursor_col = 10
   call feedkeys("ifoo barbaz\<Esc>2hRxy\<C-N>", "tx")
 
   " Insert mode
-  let b:info_compl_line = "foo bar"
-  let b:expn_compl_line = "foo "
-  let b:info_cursor_col = 8
-  let b:expn_cursor_col = 5
+  let b:compl_line = "foo bar"
+  let b:cursor_col = 8
   call feedkeys("Sfoo bar\<C-N>", "tx")
 
   set completeopt=longest
@@ -4128,7 +4128,6 @@ func Test_autocomplete_completeopt_preinsert()
   endfunc
   set omnifunc=Omni_test complete+=o
   set completeopt=preinsert autocomplete
-  " set completeopt=preinsert,menuone autocomplete
   func GetLine()
     let g:line = getline('.')
     let g:col = col('.')
diff --git a/src/version.c b/src/version.c
index 48241d3ddc..01a0dd08a5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1825,
 /**/
     1824,
 /**/