patch 9.0.0321: cannot use the message popup window directly

Problem:    Cannot use the message popup window directly.
Solution:   Add ":echowindow".
diff --git a/src/testdir/dumps/Test_echowindow_1.dump b/src/testdir/dumps/Test_echowindow_1.dump
new file mode 100644
index 0000000..8c3db07
--- /dev/null
+++ b/src/testdir/dumps/Test_echowindow_1.dump
@@ -0,0 +1,8 @@
+>s+0&#ffffff0|o|m|e| |t|e|x|t| @65
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|═+0#e000002&@74
+|f|i|r|s|t| |l|i|n|e| @64
+| +0#0000000&@56|1|,|1| @10|A|l@1| 
diff --git a/src/testdir/dumps/Test_echowindow_2.dump b/src/testdir/dumps/Test_echowindow_2.dump
new file mode 100644
index 0000000..f8bea0c
--- /dev/null
+++ b/src/testdir/dumps/Test_echowindow_2.dump
@@ -0,0 +1,8 @@
+>s+0&#ffffff0|o|m|e| |t|e|x|t| @65
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|═+0#e000002&@74
+|f|i|r|s|t| |l|i|n|e| @64
+|s|e|c|o|n|d| |l|i|n|e| @63
+| +0#0000000&@56|1|,|1| @10|A|l@1| 
diff --git a/src/testdir/dumps/Test_echowindow_3.dump b/src/testdir/dumps/Test_echowindow_3.dump
new file mode 100644
index 0000000..5d84236
--- /dev/null
+++ b/src/testdir/dumps/Test_echowindow_3.dump
@@ -0,0 +1,8 @@
+>s+0&#ffffff0|o|m|e| |t|e|x|t| @65
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+| +0#0000000&@56|1|,|1| @10|A|l@1| 
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 8683adc..2c89748 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -537,5 +537,30 @@
   set cmdheight&
 endfunc
 
+func Test_echowindow()
+  CheckScreendump
+
+  let lines =<< trim END
+      call setline(1, 'some text')
+      func ShowMessage(arg)
+        echowindow a:arg
+      endfunc
+      echowindow 'first line'
+  END
+  call writefile(lines, 'XtestEchowindow')
+  let buf = RunVimInTerminal('-S XtestEchowindow', #{rows: 8})
+  call VerifyScreenDump(buf, 'Test_echowindow_1', {})
+
+  call term_sendkeys(buf, ":call ShowMessage('second line')\<CR>")
+  call VerifyScreenDump(buf, 'Test_echowindow_2', {})
+
+  call term_sendkeys(buf, ":call popup_clear()\<CR>")
+  call VerifyScreenDump(buf, 'Test_echowindow_3', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('XtestEchowindow')
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab