Draw box if OverviewMap is added to an existing map

src/ol/control/OverviewMap.js

@@ -306,6 +306,10 @@ class OverviewMap extends Control {
           this.resetExtent_();
         }
       }
+
+      if (!this.ovmap_.isRendered()) {
+        this.updateBoxAfterOvmapIsRendered_();
+      }
     }
   }
 
@@ -519,6 +523,24 @@ class OverviewMap extends Control {
     }
   }
 
+  /**
+   * @private
+   */
+  updateBoxAfterOvmapIsRendered_() {
+    if (this.ovmapPostrenderKey_) {
+      return;
+    }
+    this.ovmapPostrenderKey_ = listenOnce(
+      this.ovmap_,
+      MapEventType.POSTRENDER,
+      function (event) {
+        delete this.ovmapPostrenderKey_;
+        this.updateBox_();
+      },
+      this
+    );
+  }
+
   /**
    * @param {MouseEvent} event The event to handle
    * @private
@@ -551,14 +573,7 @@ class OverviewMap extends Control {
       }
       ovmap.updateSize();
       this.resetExtent_();
-      listenOnce(
+      this.updateBoxAfterOvmapIsRendered_();
-        ovmap,
-        MapEventType.POSTRENDER,
-        function (event) {
-          this.updateBox_();
-        },
-        this
-      );
     }
   }