diff --git a/src/video/cocoa/cocoa_wnd.h b/src/video/cocoa/cocoa_wnd.h
--- a/src/video/cocoa/cocoa_wnd.h
+++ b/src/video/cocoa/cocoa_wnd.h
@@ -55,6 +55,8 @@ extern NSString *OTTDMainLaunchGameEngin
 @end
 
 
+extern bool _allow_hidpi_window;
+
 bool CocoaSetupApplication();
 void CocoaExitApplication();