diff --git a/src/insexpand.c b/src/insexpand.c
index 286fc0b472..c231aa41b8 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -5104,7 +5104,7 @@ get_next_default_completion(ins_compl_next_state_T *st, pos_T *start_pos)
 	    ptr = ins_compl_get_next_word_or_line(st->ins_buf,
 		    st->cur_match_pos, &len, &cont_s_ipos);
 	if (ptr == NULL || (ins_compl_has_preinsert()
-		    && STRCMP(ptr, compl_pattern.string) == 0))
+		    && STRCMP(ptr, ins_compl_leader()) == 0))
 	    continue;
 
 	if (is_nearest_active() && in_curbuf)
@@ -6222,6 +6222,7 @@ ins_compl_next(
 		    || (compl_autocomplete && !ins_compl_has_preinsert());
     int	    compl_fuzzy_match = (cur_cot_flags & COT_FUZZY) != 0;
     int	    compl_preinsert = ins_compl_has_preinsert();
+    int	    has_autocomplete_delay = (compl_autocomplete && p_acl > 0);
 
     // When user complete function return -1 for findstart which is next
     // time of 'always', compl_shown_match become NULL.
@@ -6265,7 +6266,11 @@ ins_compl_next(
 
     // Insert the text of the new completion, or the compl_leader.
     if (!started && ins_compl_preinsert_longest())
+    {
 	ins_compl_insert(TRUE, TRUE);
+	if (has_autocomplete_delay)
+	    update_screen(0);  // Show the inserted text right away
+    }
     else if (compl_no_insert && !started && !compl_preinsert)
     {
 	ins_compl_insert_bytes(compl_orig_text.string + get_compl_len(), -1);
@@ -6291,7 +6296,7 @@ ins_compl_next(
 	// may undisplay the popup menu first
 	ins_compl_upd_pum();
 
-	if (pum_enough_matches())
+	if (pum_enough_matches() && !has_autocomplete_delay)
 	    // Will display the popup menu, don't redraw yet to avoid flicker.
 	    pum_call_update_screen();
 	else
@@ -6299,16 +6304,19 @@ ins_compl_next(
 	    // inserted.
 	    update_screen(0);
 
-	// display the updated popup menu
-	ins_compl_show_pum();
-#ifdef FEAT_GUI
-	if (gui.in_use)
+	if (!has_autocomplete_delay)
 	{
-	    // Show the cursor after the match, not after the redrawn text.
-	    setcursor();
-	    out_flush_cursor(FALSE, FALSE);
-	}
+	    // display the updated popup menu
+	    ins_compl_show_pum();
+#ifdef FEAT_GUI
+	    if (gui.in_use)
+	    {
+		// Show the cursor after the match, not after the redrawn text.
+		setcursor();
+		out_flush_cursor(FALSE, FALSE);
+	    }
 #endif
+	}
 
 	// Delete old text to be replaced, since we're still searching and
 	// don't want to match ourselves!
@@ -7362,6 +7370,12 @@ ins_complete(int c, int enable_pum)
 	{
 	    if (char_avail())
 	    {
+		if (ins_compl_preinsert_effect()
+			&& ins_compl_win_active(curwin))
+		{
+		    ins_compl_delete(); // Remove pre-inserted text
+		    compl_ins_end_col = compl_col;
+		}
 		ins_compl_restart();
 		compl_interrupted = TRUE;
 		break;
diff --git a/src/testdir/dumps/Test_autocompletedelay_longest_1.dump b/src/testdir/dumps/Test_autocompletedelay_longest_1.dump
new file mode 100644
index 0000000000..10a50793cc
--- /dev/null
+++ b/src/testdir/dumps/Test_autocompletedelay_longest_1.dump
@@ -0,0 +1,10 @@
+|a+0&#ffffff0|u|t|o|c|o|m|p|l|e|t|e| @62
+|a|u|t|o|c|o|m|x@2| @64
+|a|u|t|o|c>o+0#00e0003&|m| +0#0000000&@67
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|-+2#0000000&@1| |I|N|S|E|R|T| |-@1| +0&&@44|3|,|1| @10|T|o|p| 
diff --git a/src/testdir/dumps/Test_autocompletedelay_longest_2.dump b/src/testdir/dumps/Test_autocompletedelay_longest_2.dump
new file mode 100644
index 0000000000..19a83bfa78
--- /dev/null
+++ b/src/testdir/dumps/Test_autocompletedelay_longest_2.dump
@@ -0,0 +1,10 @@
+|a+0&#ffffff0|u|t|o|c|o|m|p|l|e|t|e| @62
+|a|u|t|o|c|o|m|x@2| @64
+|a|u|t|o|c>o+0#00e0003&|m| +0#0000000&@67
+|a+0#0000001#ffd7ff255|u|t|o|c|o|m|x@2| @4| +0#4040ff13#ffffff0@59
+|a+0#0000001#ffd7ff255|u|t|o|c|o|m|p|l|e|t|e| @2| +0#4040ff13#ffffff0@59
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|-+2#0000000&@1| |I|N|S|E|R|T| |-@1| +0&&@44|3|,|1| @10|T|o|p| 
diff --git a/src/testdir/dumps/Test_autocompletedelay_longest_3.dump b/src/testdir/dumps/Test_autocompletedelay_longest_3.dump
new file mode 100644
index 0000000000..52bda2c30c
--- /dev/null
+++ b/src/testdir/dumps/Test_autocompletedelay_longest_3.dump
@@ -0,0 +1,10 @@
+|a+0&#ffffff0|u|t|o|c|o|m|p|l|e|t|e| @62
+|a|u|t|o|c|o|m|x@2| @64
+|a|u>t+0#00e0003&|o|c|o|m| +0#0000000&@67
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|-+2#0000000&@1| |I|N|S|E|R|T| |-@1| +0&&@44|3|,|1| @10|A|l@1| 
diff --git a/src/testdir/dumps/Test_autocompletedelay_longest_4.dump b/src/testdir/dumps/Test_autocompletedelay_longest_4.dump
new file mode 100644
index 0000000000..09a8514da0
--- /dev/null
+++ b/src/testdir/dumps/Test_autocompletedelay_longest_4.dump
@@ -0,0 +1,10 @@
+|a+0&#ffffff0|u|t|o|c|o|m|p|l|e|t|e| @62
+|a|u|t|o|c|o|m|x@2| @64
+|a|u>t+0#00e0003&|o|c|o|m| +0#0000000&@67
+|a+0#0000001#ffd7ff255|u|t|o|c|o|m|x@2| @4| +0#4040ff13#ffffff0@59
+|a+0#0000001#ffd7ff255|u|t|o|c|o|m|p|l|e|t|e| @2| +0#4040ff13#ffffff0@59
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|-+2#0000000&@1| |I|N|S|E|R|T| |-@1| +0&&@44|3|,|1| @10|A|l@1| 
diff --git a/src/testdir/dumps/Test_autocompletedelay_preinsert_1.dump b/src/testdir/dumps/Test_autocompletedelay_preinsert_1.dump
new file mode 100644
index 0000000000..3b1bc4bbd5
--- /dev/null
+++ b/src/testdir/dumps/Test_autocompletedelay_preinsert_1.dump
@@ -0,0 +1,10 @@
+|a+0&#ffffff0|u|t|o|c|o|m|p|l|e|t|e| @62
+|a|u|t|o|c|o|m|x@2| @64
+|a|u>t+0#00e0003&|o|c|o|m|p|l|e|t|e| +0#0000000&@62
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|-+2#0000000&@1| |I|N|S|E|R|T| |-@1| +0&&@44|3|,|1| @10|A|l@1| 
diff --git a/src/testdir/dumps/Test_autocompletedelay_preinsert_2.dump b/src/testdir/dumps/Test_autocompletedelay_preinsert_2.dump
new file mode 100644
index 0000000000..17252dfa3e
--- /dev/null
+++ b/src/testdir/dumps/Test_autocompletedelay_preinsert_2.dump
@@ -0,0 +1,10 @@
+|a+0&#ffffff0|u|t|o|c|o|m|p|l|e|t|e| @62
+|a|u|t|o|c|o|m|x@2| @64
+|a|u>t+0#00e0003&|o|c|o|m|p|l|e|t|e| +0#0000000&@62
+|a+0#0000001#e0e0e08|u|t|o|c|o|m|p|l|e|t|e| @2| +0#4040ff13#ffffff0@59
+|a+0#0000001#ffd7ff255|u|t|o|c|o|m|x@2| @4| +0#4040ff13#ffffff0@59
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|-+2#0000000&@1| |I|N|S|E|R|T| |-@1| +0&&@44|3|,|1| @10|A|l@1| 
diff --git a/src/testdir/test_ins_complete.vim b/src/testdir/test_ins_complete.vim
index e7346b8b58..b10e07433a 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -5756,7 +5756,7 @@ func Test_autocomplete_longest()
   call test_override("char_avail", 1)
   new
   inoremap <buffer><F5> <C-R>=GetLine()<CR>
-  set completeopt=longest autocomplete
+  set completeopt+=longest autocomplete
   call setline(1, ["foobar", "foozbar"])
   call feedkeys("Go\<ESC>", 'tx')
 
@@ -5996,4 +5996,45 @@ func Test_refresh_always_with_fuzzy()
   call test_override("char_avail", 0)
 endfunc
 
+func Test_autocompletedelay_longest_preinsert()
+  CheckScreendump
+  let lines =<< trim [SCRIPT]
+    call setline(1, ['autocomplete', 'autocomxxx'])
+    set autocomplete completeopt+=longest autocompletedelay=500
+  [SCRIPT]
+  call writefile(lines, 'XTest_autocompletedelay', 'D')
+  let buf = RunVimInTerminal('-S XTest_autocompletedelay', {'rows': 10})
+
+  " No spurious characters when autocompletedelay is in effect
+  call term_sendkeys(buf, "Goau")
+  sleep 10m
+  call term_sendkeys(buf, "toc")
+  sleep 100m
+  call VerifyScreenDump(buf, 'Test_autocompletedelay_longest_1', {})
+  sleep 500m
+  call VerifyScreenDump(buf, 'Test_autocompletedelay_longest_2', {})
+
+  " Deleting a char should still show longest text
+  call term_sendkeys(buf, "\<Esc>Saut")
+  sleep 10m
+  call term_sendkeys(buf, "\<BS>")
+  sleep 100m
+  call VerifyScreenDump(buf, 'Test_autocompletedelay_longest_3', {})
+  sleep 500m
+  call VerifyScreenDump(buf, 'Test_autocompletedelay_longest_4', {})
+
+  " Preinsert
+  call term_sendkeys(buf, "\<Esc>:set completeopt& completeopt+=preinsert\<CR>")
+
+  " Show preinserted text right away but display popup later
+  call term_sendkeys(buf, "\<Esc>Sau")
+  sleep 100m
+  call VerifyScreenDump(buf, 'Test_autocompletedelay_preinsert_1', {})
+  sleep 500m
+  call VerifyScreenDump(buf, 'Test_autocompletedelay_preinsert_2', {})
+
+  call term_sendkeys(buf, "\<esc>")
+  call StopVimInTerminal(buf)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab nofoldenable
diff --git a/src/version.c b/src/version.c
index 431536543a..773c66b1b6 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 */
+/**/
+    1820,
 /**/
     1819,
 /**/