Fix balloon API in MacVim GUI

* Fix the position of calling setLastToolTip: in gui_mch_retain_font()
* Set balloonEval->msg in gui_mch_post_balloon()

This fix makes Test_balloon_show_gui pass.

Author: ichizok

src/MacVim/MMBackend.m

@@ -3359,14 +3359,14 @@ - (void)bevalCallback:(id UNUSED)sender
         // variable.  (The reason we need to know is due to how the Cocoa tool
         // tips work: if there is no tool tip we must set it to nil explicitly
         // or it might never go away.)
-        [self setLastToolTip:nil];
-
         (*balloonEval->msgCB)(balloonEval, 0);
 
         [self queueMessage:SetTooltipMsgID properties:
             [NSDictionary dictionaryWithObject:(lastToolTip ? lastToolTip : @"")
                                         forKey:@"toolTip"]];
         [self flushQueue:YES];
+
+        [self setLastToolTip:nil];
     }
 }
 #endif

src/MacVim/gui_macvim.m

@@ -2514,8 +2514,11 @@
  * Show a balloon with "mesg".
  */
     void
-gui_mch_post_balloon(BalloonEval *beval UNUSED, char_u *mesg)
+gui_mch_post_balloon(BalloonEval *beval, char_u *mesg)
 {
+    vim_free(beval->msg);
+    beval->msg = mesg == NULL ? NULL : vim_strsave(mesg);
+
     NSString *toolTip = [NSString stringWithVimString:mesg];
     [[MMBackend sharedInstance] setLastToolTip:toolTip];
 }

src/testdir/test_balloon_gui.vim

@@ -4,8 +4,6 @@ source check.vim
 CheckGui
 CheckFeature balloon_eval
 
-if !has('gui_macvim') " See https://github.com/macvim-dev/macvim/issues/902
-
 func Test_balloon_show_gui()
   let msg = 'this this this this'
   call balloon_show(msg)
@@ -20,5 +18,4 @@ func Test_balloon_show_gui()
   call balloon_show('')
 endfunc
 
-endif " !has('gui_macvim')
 " vim: shiftwidth=2 sts=2 expandtab