Bug 1801678 - Compute the right insertion point for a browser action widget when the toolbar is overflowed. r=Gijs

We have a function, OverflowableToolbar.findOverflowedInsertionPoints, whose job it is to compute
the right insertion point for a widget if the widget is being inserted into an overflowable
toolbar.

That function failed to account for the possibility that the widget was a browser action, and
would return the #defaultList instead of the #webExtList if the item was overflowing. This
patch checks the ID of the widget first, and then supplies the correct list.

Differential Revision: https://phabricator.services.mozilla.com/D162586

browser/components/customizableui/CustomizableUI.jsm

@@ -5611,9 +5611,13 @@ class OverflowableToolbar {
       }
     }
 
+    let overflowList = CustomizableUI.isWebExtensionWidget(aNode.id)
+      ? this.#webExtList
+      : this.#defaultList;
+
     let containerForAppending =
       this.#overflowedInfo.size && newNodeCanOverflow
-        ? this.#defaultList
+        ? overflowList
         : this.#target;
     return [containerForAppending, null];
   }