fix(ui2): prevent flicker when entering pager from expanded cmdline (#38662)

fix(ui2): flicker when entering pager from expanded cmdline #38639

Problem:  'showcmd' causes flickering when pressing "g<" to enter the
          pager when the cmdline is expanded for messages.
          Initial keypress for an incomplete mapping is not giving 'showcmd'
          feedback while cmdline is expanded for messages (which is only
          dismissed upon the vim.on_key callback after 'timeoutlen').

Solution: Delay dismissing expanded cmdline when vim.on_key() callback
          receives "g".
          Place 'showcmd' "last" virtual text during expanded cmdline.
(cherry picked from commit 75e5e37942)

Co-authored-by: luukvbaal <luukvbaal@gmail.com>

runtime/lua/vim/_core/ui2/cmdline.lua

@@ -96,7 +96,7 @@ function M.cmdline_show(content, pos, firstc, prompt, indent, level, hl_id)
   if M.level == 0 and ui.msg.cmd_on_key then
     M.expand, M.dialog, ui.msg.cmd_on_key = 1, true, nil
     api.nvim_win_set_config(ui.wins.cmd, { border = 'none' })
-    ui.msg.expand_msg('cmd')
+    ui.msg.expand_msg('cmd', 'dialog')
   elseif ui.msg.cmd.msg_row ~= -1 and M.expand == 0 then
     ui.msg.msg_clear()
   end

runtime/lua/vim/_core/ui2/messages.lua

@@ -86,7 +86,12 @@ end
 ---@param type 'last'|'msg'|'top'|'bot'
 ---@param tgt? 'cmd'|'msg'|'dialog'
 local function set_virttext(type, tgt)
-  if (type == 'last' and (ui.cmdheight == 0 or M.virt.delayed)) or M.cmd_on_key then
+  if
+    -- Cannot show with 0 'cmdheight' and delay for error.
+    (type == 'last' and (ui.cmdheight == 0 or M.virt.delayed))
+    -- Do not overwrite temporary spill indicator during expanded cmdline.
+    or (M.cmd_on_key and type == 'msg')
+  then
     return -- Don't show virtual text while cmdline is expanded or delaying for error.
   end
 
@@ -192,12 +197,12 @@ end
 
 local hlopts = { undo_restore = false, invalidate = true, priority = 1 }
 --- Move messages to expanded cmdline, dialog or pager to show in full.
-function M.expand_msg(src)
+function M.expand_msg(src, tgt)
   -- Copy and clear message from src to enlarged cmdline that is dismissed by any
   -- key press. Append to pager instead if it isn't hidden or we want to enter it
   -- after cmdline was entered during expanded cmdline.
   local hidden = api.nvim_win_get_config(ui.wins.pager).hide
-  local tgt = (src == 'dialog' or not hidden) and 'pager' or ui.cmd.expand > 0 and 'dialog' or 'cmd'
+  tgt = tgt or not hidden and 'pager' or 'cmd'
   if tgt ~= src then
     local srow = hidden and 0 or api.nvim_buf_line_count(ui.bufs.pager)
     local opts = { details = true, type = 'highlight' }
@@ -428,7 +433,7 @@ function M.msg_show(kind, content, replace_last, _, append, id, trigger)
     -- When message was emitted below an already expanded cmdline, move and route to pager.
     tgt = ui.cmd.expand > 0 and 'pager' or tgt
     if ui.cmd.expand == 1 then
-      M.expand_msg('dialog')
+      M.expand_msg('dialog', 'pager')
     end
     ui.cmd.expand = ui.cmd.expand + (ui.cmd.expand > 0 and 1 or 0)
 
@@ -476,7 +481,7 @@ end
 ---
 ---@param content MsgContent
 function M.msg_ruler(content)
-  M.virt.last[M.virt.idx.ruler] = ui.cmd.level > 0 and {} or content
+  M.virt.last[M.virt.idx.ruler] = (ui.cmd.level > 0 or M.cmd_on_key) and {} or content
   set_virttext('last')
 end
 
@@ -493,7 +498,6 @@ function M.msg_history_show(entries, prev_cmd)
   -- Showing output of previous command, clear in case still visible.
   if M.cmd_on_key or prev_cmd then
     M.msg_clear()
-    api.nvim_feedkeys(vim.keycode('<Esc>'), 'n', false)
   end
 
   api.nvim_buf_set_lines(ui.bufs.pager, 0, -1, false, {})
@@ -505,28 +509,33 @@ function M.msg_history_show(entries, prev_cmd)
   M.set_pos('pager')
 end
 
-local cmd_on_key = function(_, typed)
+local typed_g = false
+local cmd_on_key = function(key, typed)
   typed = typed and fn.keytrans(typed)
-  if not typed or typed == '<MouseMove>' or typed == ':' then
-    if typed == ':' then
-      vim.on_key(nil, ui.ns)
-    end
+  -- Don't dismiss for non-typed keys and mouse movement. When 'g' is passed (typed
+  -- or mapped), wait until the next key to avoid flickering when the pager is opened.
+  if not typed_g and (not typed or typed == '<MouseMove>' or typed == 'g' or key == 'g') then
+    typed_g = typed == 'g' or key == 'g'
     return
   end
   vim.on_key(nil, ui.ns)
-  M.cmd_on_key, M.cmd.ids = nil, {}
+  if typed == ':' then
+    return -- Keep expanded messages open until cmdline closes.
+  end
 
   -- Check if window was entered and reopen with original config.
-  local entered = typed == '<CR>' and not api.nvim_get_mode().mode:match('[it]')
-    or typed:find('LeftMouse') and fn.getmousepos().winid == ui.wins.cmd
+  local mode = not api.nvim_get_mode().mode:match('[it]')
+  local entered = mode and (typed == '<CR>' or typed_g and (typed == '<lt>' or key == '<'))
+    or (typed:find('LeftMouse') and fn.getmousepos().winid == ui.wins.cmd)
+  if entered then
+    M.expand_msg('cmd', 'pager')
+  end
   pcall(api.nvim_win_close, ui.wins.cmd, true)
   ui.check_targets()
-
-  -- Show or clear the message depending on if the pager was opened.
-  if entered then
-    api.nvim_command('norm! g<')
-  end
   set_virttext('msg')
+  api.nvim__redraw({ flush = true })
+
+  typed_g, M.cmd_on_key, M.cmd.ids = false, nil, {}
   return entered and ''
 end
 
@@ -593,6 +602,8 @@ local function enter_pager()
   in_pager, was_cmdwin = true, fn.getcmdwintype()
   if was_cmdwin ~= '' then
     api.nvim_command('quit')
+  elseif M.cmd_on_key then
+    api.nvim_feedkeys(vim.keycode('<Esc>'), 'n', false)
   end
   -- Cmdwin is closed one event iteration later so schedule in case it was open.
   vim.schedule(function()

test/functional/ui/messages2_spec.lua

@@ -39,7 +39,7 @@ describe('messages2', function()
   end)
 
   it('multiline messages and pager', function()
-    command('echo "foo\nbar"')
+    command('set ruler showcmd noshowmode | echo "foo\nbar"')
     screen:expect([[
       ^                                                     |
       {1:~                                                    }|*10
@@ -47,8 +47,15 @@ describe('messages2', function()
       foo                                                  |
       bar                                                  |
     ]])
-    command('set ruler showcmd noshowmode')
-    feed('g<lt>')
+    feed('g')
+    screen:expect([[
+      ^                                                     |
+      {1:~                                                    }|*10
+      {3:                                                     }|
+      foo                                                  |
+      bar                                       g          |
+    ]])
+    feed('<lt>')
     screen:expect([[
                                                            |
       {1:~                                                    }|*9
@@ -76,7 +83,7 @@ describe('messages2', function()
     screen:expect([[
       ^                                                     |
       {1:~                                                    }|*12
-      foo [+29]                           0,0-1         All|
+      foo [+29]                                            |
     ]])
     command('echo "foo"')
     -- New message clears spill indicator.
@@ -835,7 +842,7 @@ describe('messages2', function()
         - cCommentL                                        |
     ]])
     feed('Q')
-    screen:expect_unchanged()
+    screen:expect_unchanged(true)
     feed('<C-L>') -- close expanded cmdline
     set_msg_target_zero_ch()
     api.nvim_echo({ { 'foo' } }, true, { id = 1 })