draw selection rect only if mouse has moved

src/ui/WorldRenderer.h

@@ -55,6 +55,9 @@ struct WorldRenderFrame
     const BuildModeController&     buildMode;
     const std::vector<ActiveBeam>& beams;
     bool                           isBoxSelecting;
+    // Whether that box drag has passed the movement threshold that tells it apart
+    // from a click; until it has, the rectangle is not drawn (REQ-UI-MULTI-SELECT).
+    bool                           isBoxDragMoved;
     QPoint                         boxStartTile;
     QPoint                         boxCurrentTile;
     bool                           isDebugDrawEnabled;