diff --git a/src/viewport_func.h b/src/viewport_func.h
--- a/src/viewport_func.h
+++ b/src/viewport_func.h
@@ -26,7 +26,7 @@ void InitializeWindowViewport(Window *w,
 Viewport *IsPtInWindowViewport(const Window *w, int x, int y);
 Point TranslateXYToTileCoord(const Viewport *vp, int x, int y, bool clamp_to_map = true);
 Point GetTileBelowCursor();
-void UpdateViewportPosition(Window *w);
+void UpdateViewportPosition(Window *w, uint32_t delta_ms);
 
 bool MarkAllViewportsDirty(int left, int top, int right, int bottom);