patch 9.1.1621: flicker in popup menu during cmdline autocompletion

Problem:  When the popup menu (PUM) occupies more than half the screen
          height, it flickers whenever a character is typed or erased.
          This happens because the PUM is cleared and the screen is
          redrawn before a new PUM is rendered. The extra redraw between
          menu updates causes visible flicker.
Solution: A complete, non-hacky fix would require removing the
          CmdlineChanged event from the loop and letting autocompletion
          manage the process end-to-end. This is because screen redraws
          after any cmdline change are necessary for other features to
          work.
          This change modifies wildtrigger() so that the next typed
          character defers the screen update instead of redrawing
          immediately. This removes the intermediate redraw, eliminating
          flicker and making cmdline autocompletion feel smooth
          (Girish Palya).

Trade-offs:
This behavior change in wildtrigger() is tailored specifically for
:h cmdline-autocompletion. wildtrigger() now has no general-purpose use
outside this scenario.

closes: #17932

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/cmdexpand.c

@@ -455,9 +455,8 @@ cmdline_pum_active(void)
  * items and refresh the screen.
  */
     void
-cmdline_pum_remove(cmdline_info_T *cclp UNUSED)
+cmdline_pum_remove(cmdline_info_T *cclp UNUSED, int defer_redraw)
 {
-    int save_p_lz = p_lz;
     int	save_KeyTyped = KeyTyped;
 #ifdef FEAT_EVAL
     int	save_RedrawingDisabled = RedrawingDisabled;
@@ -468,9 +467,15 @@ cmdline_pum_remove(cmdline_info_T *cclp UNUSED)
     pum_undisplay();
     VIM_CLEAR(compl_match_array);
     compl_match_arraysize = 0;
+    if (!defer_redraw)
+    {
+	int save_p_lz = p_lz;
 	p_lz = FALSE;  // avoid the popup menu hanging around
 	update_screen(0);
 	p_lz = save_p_lz;
+    }
+    else
+	pum_call_update_screen();
     redrawcmd();
 
     // When a function is called (e.g. for 'foldtext') KeyTyped might be reset
@@ -485,7 +490,7 @@ cmdline_pum_remove(cmdline_info_T *cclp UNUSED)
     void
 cmdline_pum_cleanup(cmdline_info_T *cclp)
 {
-    cmdline_pum_remove(cclp);
+    cmdline_pum_remove(cclp, FALSE);
     wildmenu_cleanup(cclp);
 }
 
@@ -1068,7 +1073,7 @@ ExpandOne(
 
 	// The entries from xp_files may be used in the PUM, remove it.
 	if (compl_match_array != NULL)
-	    cmdline_pum_remove(get_cmdline_info());
+	    cmdline_pum_remove(get_cmdline_info(), FALSE);
     }
     xp->xp_selected = 0;
 

src/ex_getln.c

@@ -939,6 +939,7 @@ cmdline_wildchar_complete(
 	int		*wim_index_p,
 	expand_T	*xp,
 	int		*gotesc,
+	int		redraw_if_menu_empty,
 	pos_T		*pre_incsearch_pos)
 {
     int		wim_index = *wim_index_p;
@@ -991,6 +992,10 @@ cmdline_wildchar_complete(
 	else
 	    res = nextwild(xp, WILD_EXPAND_KEEP, options, escape);
 
+	// Remove popup window if no completion items are available
+	if (redraw_if_menu_empty && xp->xp_numfiles <= 0)
+	    update_screen(0);
+
 	// if interrupted while completing, behave like it failed
 	if (got_int)
 	{
@@ -1633,7 +1638,7 @@ getcmdline_int(
     int		clear_ccline)	// clear ccline first
 {
     static int	depth = 0;	    // call depth
-    int		c;
+    int		c = 0;
     int		i;
     int		j;
     int		gotesc = FALSE;		// TRUE when <ESC> just typed
@@ -1838,6 +1843,7 @@ getcmdline_int(
 	int	trigger_cmdlinechanged = TRUE;
 	int	end_wildmenu;
 	int	prev_cmdpos = ccline.cmdpos;
+	int	skip_pum_redraw = FALSE;
 
 	VIM_CLEAR(prev_cmdbuff);
 
@@ -1863,6 +1869,10 @@ getcmdline_int(
 		goto returncmd;
 	}
 
+	// Defer screen update to avoid pum flicker during wildtrigger()
+	if (c == K_WILD && firstc != '@')
+	    skip_pum_redraw = TRUE;
+
 	// Get a character.  Ignore K_IGNORE and K_NOP, they should not do
 	// anything, such as stop completion.
 	do
@@ -2002,7 +2012,12 @@ getcmdline_int(
 	if (end_wildmenu)
 	{
 	    if (cmdline_pum_active())
-		cmdline_pum_remove(&ccline);
+	    {
+		skip_pum_redraw = skip_pum_redraw && (vim_isprintc(c)
+			|| c == K_BS || c == Ctrl_H || c == K_DEL
+			|| c == K_KDEL || c == Ctrl_W || c == Ctrl_U);
+		cmdline_pum_remove(&ccline, skip_pum_redraw);
+	    }
 	    if (xpc.xp_numfiles != -1)
 		(void)ExpandOne(&xpc, NULL, NULL, 0, WILD_FREE);
 	    did_wild_list = FALSE;
@@ -2081,7 +2096,7 @@ getcmdline_int(
 	    if (c == K_WILD)
 		++emsg_silent;  // Silence the bell
 	    res = cmdline_wildchar_complete(c, firstc != '@', &did_wild_list,
-		    &wim_index, &xpc, &gotesc,
+		    &wim_index, &xpc, &gotesc, c == K_WILD,
 #ifdef FEAT_SEARCH_EXTRA
 		    &is_state.search_start
 #else
@@ -2647,7 +2662,7 @@ returncmd:
     // if certain special keys like <Esc> or <C-\> were used as wildchar. Make
     // sure to still clean up to avoid memory corruption.
     if (cmdline_pum_active())
-	cmdline_pum_remove(&ccline);
+	cmdline_pum_remove(&ccline, FALSE);
     wildmenu_cleanup(&ccline);
     did_wild_list = FALSE;
     wim_index = 0;

src/proto/cmdexpand.pro

@@ -3,7 +3,7 @@ int cmdline_fuzzy_complete(char_u *fuzzystr);
 int nextwild(expand_T *xp, int type, int options, int escape);
 void cmdline_pum_display(void);
 int cmdline_pum_active(void);
-void cmdline_pum_remove(cmdline_info_T *cclp);
+void cmdline_pum_remove(cmdline_info_T *cclp, int defer);
 void cmdline_pum_cleanup(cmdline_info_T *cclp);
 int cmdline_compl_startcol(void);
 char_u *cmdline_compl_pattern(void);

src/testdir/dumps/Test_wildtrigger_update_screen_1.dump

@@ -0,0 +1,10 @@
+| +0&#ffffff0@74
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @6| +0#0000001#ffd7ff255|a|b|c|1| @10| +0#4040ff13#ffffff0@50
+|~| @6| +0#0000001#ffd7ff255|a|b|c|2| @10| +0#4040ff13#ffffff0@50
+|~| @6| +0#0000001#ffd7ff255|a|b|c|3| @10| +0#4040ff13#ffffff0@50
+|~| @6| +0#0000001#ffd7ff255|a|b|c|4| @10| +0#4040ff13#ffffff0@50
+|~| @6| +0#0000001#ffd7ff255|a|b|c|5| @10| +0#4040ff13#ffffff0@50
+|:+0#0000000&|T|e|s|t|C|m|d| |a> @64

src/testdir/dumps/Test_wildtrigger_update_screen_2.dump

@@ -0,0 +1,10 @@
+| +0&#ffffff0@74
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @6| +0#0000001#ffd7ff255|a|b|c|1| @10| +0#4040ff13#ffffff0@50
+|~| @6| +0#0000001#ffd7ff255|a|b|c|2| @10| +0#4040ff13#ffffff0@50
+|~| @6| +0#0000001#ffd7ff255|a|b|c|3| @10| +0#4040ff13#ffffff0@50
+|~| @6| +0#0000001#ffd7ff255|a|b|c|4| @10| +0#4040ff13#ffffff0@50
+|~| @6| +0#0000001#ffd7ff255|a|b|c|5| @10| +0#4040ff13#ffffff0@50
+|:+0#0000000&|T|e|s|t|C|m|d| |a|x> @63

src/testdir/dumps/Test_wildtrigger_update_screen_3.dump

@@ -0,0 +1,10 @@
+| +0&#ffffff0@74
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|:+0#0000000&|T|e|s|t|C|m|d| |a|x> @45|0|,|0|-|1| @8|A|l@1| 

src/testdir/test_cmdline.vim

@@ -4822,4 +4822,41 @@ func Test_cmdline_changed()
   call test_override("char_avail", 0)
 endfunc
 
+func Test_wildtrigger_update_screen()
+  CheckScreendump
+  let lines =<< trim [SCRIPT]
+    command! -nargs=* -complete=customlist,TestFn TestCmd echo
+    func TestFn(cmdarg, b, c)
+      if a:cmdarg == 'ax'
+        return []
+      else
+        return map(range(1, 5), 'printf("abc%d", v:val)')
+      endif
+    endfunc
+    set wildmode=noselect,full
+    set wildoptions=pum
+    set wildmenu
+    cnoremap <F8> <C-R>=wildtrigger()[-1]<CR>
+  [SCRIPT]
+  call writefile(lines, 'XTest_wildtrigger', 'D')
+  let buf = RunVimInTerminal('-S XTest_wildtrigger', {'rows': 10})
+
+  call term_sendkeys(buf, ":TestCmd a\<F8>")
+  call VerifyScreenDump(buf, 'Test_wildtrigger_update_screen_1', {})
+
+  " Typing a character when pum is open does not close the pum window
+  " This is needed to prevent pum window from flickering during
+  " ':h cmdline-autocompletion'.
+  call term_sendkeys(buf, "x")
+  call VerifyScreenDump(buf, 'Test_wildtrigger_update_screen_2', {})
+
+  " pum window is closed when no completion candidates are available
+  call term_sendkeys(buf, "\<F8>")
+  call VerifyScreenDump(buf, 'Test_wildtrigger_update_screen_3', {})
+
+  call term_sendkeys(buf, "\<esc>")
+  call StopVimInTerminal(buf)
+  cnoremap <buffer> <F8> <C-R>=wildtrigger()[-1]<CR>
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -719,6 +719,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1621,
 /**/
     1620,
 /**/