Fix For issue #6948 (#6991)

MessageBoxes attached to a window in macOS should use modal APIs and not
use a poll/sleep pattern on the main thread. Sleeping the main thread
makes the NSWindow message loop sluggish and interferes with external
applications that need to send messages to that window, such as
VoiceOver.

src/video/cocoa/SDL_cocoamessagebox.m

@@ -31,7 +31,6 @@
     NSWindow *nswindow;
 }
 - (id)initWithParentWindow:(SDL_Window *)window;
-- (void)alertDidEnd:(NSAlert *)alert returnCode:(NSInteger)returnCode contextInfo:(void *)contextInfo;
 @end
 
 @implementation SDLMessageBoxPresenter
@@ -55,39 +54,16 @@
 - (void)showAlert:(NSAlert *)alert
 {
     if (nswindow) {
-#ifdef MAC_OS_X_VERSION_10_9
-        if ([alert respondsToSelector:@selector(beginSheetModalForWindow:completionHandler:)]) {
-            [alert beginSheetModalForWindow:nswindow
-                          completionHandler:^(NSModalResponse returnCode) {
-                            self->clicked = returnCode;
-                          }];
-        } else
-#endif
-        {
-#if MAC_OS_X_VERSION_MIN_REQUIRED < 1090
-            [alert beginSheetModalForWindow:nswindow
-                              modalDelegate:self
-                             didEndSelector:@selector(alertDidEnd:returnCode:contextInfo:)
-                                contextInfo:nil];
-#endif
-        }
-
-        while (clicked < 0) {
-            SDL_PumpEvents();
-            SDL_Delay(100);
-        }
-
+        [alert beginSheetModalForWindow:nswindow
+                      completionHandler:^(NSModalResponse returnCode) {
+                        [NSApp stopModalWithCode:returnCode];
+                      }];
+        clicked = [NSApp runModalForWindow:nswindow];
         nswindow = nil;
     } else {
         clicked = [alert runModal];
     }
 }
-
-- (void)alertDidEnd:(NSAlert *)alert returnCode:(NSInteger)returnCode contextInfo:(void *)contextInfo
-{
-    clicked = returnCode;
-}
-
 @end
 
 static void Cocoa_ShowMessageBoxImpl(const SDL_MessageBoxData *messageboxdata, int *buttonid, int *returnValue)