Bug 1588747 - Always insert the TitlebarGradientView underneath its siblings. r=spohl

Usually it covers a different area than the window's ChildView. But in some cases
it can overlap with the ChildView, and in those cases, it's better if the
ChildView is on top.

Differential Revision: https://phabricator.services.mozilla.com/D49332

--HG--
extra : moz-landing-system : lando

widget/cocoa/nsCocoaWindow.mm

@@ -3322,7 +3322,7 @@ static const NSString* kStateWantsTitleDrawn = @"wantsTitleDrawn";
   if (needTitlebarView && !mTitlebarGradientView) {
     mTitlebarGradientView = [[TitlebarGradientView alloc] initWithFrame:[self titlebarRect]];
     mTitlebarGradientView.autoresizingMask = NSViewWidthSizable | NSViewMinYMargin;
-    [self.contentView addSubview:mTitlebarGradientView];
+    [self.contentView addSubview:mTitlebarGradientView positioned:NSWindowBelow relativeTo:nil];
   } else if (!needTitlebarView && mTitlebarGradientView) {
     [mTitlebarGradientView removeFromSuperview];
     [mTitlebarGradientView release];