patch 9.0.0356: :echowindow sets the in_echowindow flag too early

Problem:    :echowindow sets the in_echowindow flag too early.
Solution:   Set in_echowindow only when outputting the text. (Yasuhiro
            Matsumoto, closes #11033)

src/eval.c

@@ -6728,10 +6728,6 @@ ex_execute(exarg_T *eap)
     long	start_lnum = SOURCING_LNUM;
 
     ga_init2(&ga, 1, 80);
-#ifdef HAS_MESSAGE_WINDOW
-    if (eap->cmdidx == CMD_echowindow)
-	start_echowindow();
-#endif
 
     if (eap->skip)
 	++emsg_skip;
@@ -6796,11 +6792,21 @@ ex_execute(exarg_T *eap)
 	    msg_sb_eol();
 	}
 
-	if (eap->cmdidx == CMD_echomsg || eap->cmdidx == CMD_echowindow)
+	if (eap->cmdidx == CMD_echomsg)
 	{
 	    msg_attr(ga.ga_data, echo_attr);
 	    out_flush();
 	}
+	else if (eap->cmdidx == CMD_echowindow)
+	{
+#ifdef HAS_MESSAGE_WINDOW
+	    start_echowindow();
+#endif
+	    msg_attr(ga.ga_data, echo_attr);
+#ifdef HAS_MESSAGE_WINDOW
+	    end_echowindow();
+#endif
+	}
 	else if (eap->cmdidx == CMD_echoconsole)
 	{
 	    ui_write(ga.ga_data, (int)STRLEN(ga.ga_data), TRUE);
@@ -6832,10 +6838,6 @@ ex_execute(exarg_T *eap)
 
     if (eap->skip)
 	--emsg_skip;
-#ifdef HAS_MESSAGE_WINDOW
-    if (eap->cmdidx == CMD_echowindow)
-	end_echowindow();
-#endif
     set_nextcmd(eap, arg);
 }
 

src/testdir/dumps/Test_echowin_eval.dump

@@ -0,0 +1,8 @@
+> +0&#ffffff0@74
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|═+0#e000002&@74
+|t|e|s|t| @70
+|1+0#0000000&|2|3| @53|0|,|0|-|1| @8|A|l@1| 

src/testdir/test_messages.vim

@@ -420,5 +420,25 @@ func Test_echowindow()
   call delete('XtestEchowindow')
 endfunc
 
+" messages window should not be used while evaluating the :echowin argument
+func Test_echowin_eval()
+  CheckScreendump
+
+  let lines =<< trim END
+      func ShowMessage()
+        echo 123
+        return 'test'
+      endfunc
+      echowindow ShowMessage()
+  END
+  call writefile(lines, 'XtestEchowindow')
+  let buf = RunVimInTerminal('-S XtestEchowindow', #{rows: 8})
+  call VerifyScreenDump(buf, 'Test_echowin_eval', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('XtestEchowindow')
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -707,6 +707,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    356,
 /**/
     355,
 /**/