Squelch context menus on the OSX App List when both buttons are pressed.

ui/app_list/cocoa/apps_grid_view_item.mm

Index: ui/app_list/cocoa/apps_grid_view_item.mm
diff --git a/ui/app_list/cocoa/apps_grid_view_item.mm b/ui/app_list/cocoa/apps_grid_view_item.mm
index e3101efcbbbf33ccf768f134ea1698b19b20c0da..fec60aae15c774fe22240e6ff4e2b0541ab9feff 100644
--- a/ui/app_list/cocoa/apps_grid_view_item.mm
+++ b/ui/app_list/cocoa/apps_grid_view_item.mm
@@ -319,6 +319,10 @@ void ItemModelObserverBridge::ItemPercentDownloadedChanged() {
 }
 
 - (NSMenu*)contextMenu {
+  // Don't show the menu if button is already held down, e.g. with a left-click.
+  if ([[[self button] cell] isHighlighted])
+    return nil;
+
   [self setSelected:YES];
   return observerBridge_->GetContextMenu();
 }