macosx: disable native fullscreen on El Capitan

Native fullscreen seems to be on by default on El Capitan, even
if the corresponding flag is not set.
This implicitely disables the new split view feature if native
fullscreen is disabled.

close #15491

modules/gui/macosx/Windows.m

@@ -334,6 +334,9 @@
 
     if (b_nativeFullscreenMode) {
         [self setCollectionBehavior: NSWindowCollectionBehaviorFullScreenPrimary];
+    } else if (OSX_EL_CAPITAN) {
+        // Native fullscreen seems to be default on El Capitan, this disables it explicitely
+        [self setCollectionBehavior: NSWindowCollectionBehaviorFullScreenAuxiliary];
     }
 
     [super awakeFromNib];