patch 9.0.0447: using :echowin while at the hit-enter prompt causes problems

Problem:    Using :echowin while at the hit-enter prompt causes problems.
Solution:   Do not prompt for :echowin.  Postpone showing the message window.
            Start the timer when the window is displayed.
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 4a2deae..6dc9a5b 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -401,6 +401,28 @@
           echowindow 'line' n
         endfor
       endfunc
+
+      def TwoMessages()
+        popup_clear()
+        set cmdheight=2
+        redraw
+        timer_start(100, (_) => {
+            echowin 'message'
+          })
+        echo 'one'
+        echo 'two'
+      enddef
+
+      def ThreeMessages()
+        popup_clear()
+        redraw
+        timer_start(100, (_) => {
+            echowin 'later message'
+          })
+        echo 'one'
+        echo 'two'
+        echo 'three'
+      enddef
   END
   call writefile(lines, 'XtestEchowindow')
   let buf = RunVimInTerminal('-S XtestEchowindow', #{rows: 8})
@@ -415,6 +437,16 @@
   call term_sendkeys(buf, ":call ManyMessages()\<CR>")
   call VerifyScreenDump(buf, 'Test_echowindow_4', {})
 
+  call term_sendkeys(buf, ":call TwoMessages()\<CR>")
+  call VerifyScreenDump(buf, 'Test_echowindow_5', {})
+
+  call term_sendkeys(buf, ":call ThreeMessages()\<CR>")
+  sleep 120m
+  call VerifyScreenDump(buf, 'Test_echowindow_6', {})
+
+  call term_sendkeys(buf, "\<CR>")
+  call VerifyScreenDump(buf, 'Test_echowindow_7', {})
+
   " clean up
   call StopVimInTerminal(buf)
   call delete('XtestEchowindow')