diff --git a/src/viewport_func.h b/src/viewport_func.h
--- a/src/viewport_func.h
+++ b/src/viewport_func.h
@@ -31,6 +31,7 @@ bool MarkAllViewportsDirty(int left, int
 
 bool DoZoomInOutWindow(ZoomStateChange how, Window *w);
 void ZoomInOrOutToCursorWindow(bool in, Window * w);
+void ConstrainAllViewportsZoom();
 Point GetTileZoomCenterWindow(bool in, Window * w);
 void FixTitleGameZoom(int zoom_adjust = 0);
 void HandleZoomMessage(Window *w, const Viewport *vp, byte widget_zoom_in, byte widget_zoom_out);