vim-patch:9.0.0595: extra newline in messages after a verbose shell message (#20359)

Problem:    Extra newline in messages after a verbose shell message.
Solution:   Output the newline with msg_putchar_attr(). (closes vim/vim#11233)
            Make it possible to filter a screendump before comparing it.
1190139ed0

Cherry-pick Test_message_more_scrolledback() from patch 9.0.0592 because
Nvim already behaves as intended.

src/nvim/message.c

@@ -2315,7 +2315,7 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
   if (t_col > 0) {
     t_puts(&t_col, t_s, s, attr);
   }
-  if (p_more && !recurse) {
+  if (p_more && !recurse && !(s == sb_str + 1 && *sb_str == '\n')) {
     store_sb_text((char **)&sb_str, (char *)s, attr, &sb_col, false);
   }
 

src/nvim/testdir/test_messages.vim

@@ -316,6 +316,66 @@ func Test_message_more()
   call StopVimInTerminal(buf)
 endfunc
 
+" Test more-prompt scrollback
+func Test_message_more_scrollback()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+      set t_ut=
+      hi Normal ctermfg=15 ctermbg=0
+      for i in range(100)
+          echo i
+      endfor
+  END
+  call writefile(lines, 'XmoreScrollback', 'D')
+  let buf = RunVimInTerminal('-S XmoreScrollback', {'rows': 10})
+  call VerifyScreenDump(buf, 'Test_more_scrollback_1', {})
+
+  call term_sendkeys(buf, 'f')
+  call TermWait(buf)
+  call term_sendkeys(buf, 'b')
+  call VerifyScreenDump(buf, 'Test_more_scrollback_2', {})
+
+  call term_sendkeys(buf, 'q')
+  call TermWait(buf)
+  call StopVimInTerminal(buf)
+endfunc
+
+" Test verbose message before echo command
+func Test_echo_verbose_system()
+  CheckRunVimInTerminal
+  CheckUnix
+
+  let buf = RunVimInTerminal('', {'rows': 10})
+  call term_sendkeys(buf, ":4 verbose echo system('seq 20')\<CR>")
+  " Note that the screendump is filtered to remove the name of the temp file
+  call VerifyScreenDump(buf, 'Test_verbose_system_1', {})
+
+  " display a page and go back, results in exactly the same view
+  call term_sendkeys(buf, ' ')
+  call TermWait(buf)
+  call term_sendkeys(buf, 'b')
+  call VerifyScreenDump(buf, 'Test_verbose_system_1', {})
+
+  " do the same with 'cmdheight' set to 2
+  call term_sendkeys(buf, 'q')
+  call TermWait(buf)
+  call term_sendkeys(buf, ":set ch=2\<CR>")
+  call TermWait(buf)
+  call term_sendkeys(buf, ":4 verbose echo system('seq 20')\<CR>")
+  call VerifyScreenDump(buf, 'Test_verbose_system_2', {})
+
+  call term_sendkeys(buf, ' ')
+  call TermWait(buf)
+  call term_sendkeys(buf, 'b')
+  call VerifyScreenDump(buf, 'Test_verbose_system_2', {})
+
+  call term_sendkeys(buf, 'q')
+  call TermWait(buf)
+  call StopVimInTerminal(buf)
+endfunc
+
+
 func Test_ask_yesno()
   CheckRunVimInTerminal
   let buf = RunVimInTerminal('', {'rows': 6})