Liferay edit mode: content not visible when adding same portlet multiple times on the page

[mcollovati]

When adding the same portlet twice on a Liferay Content Page in edit mode, the first instance has visible contents, but the second one is blank.

As a workaround a user can simply refresh the page, and all portlet instances are correctly rendered.

May be related to #211

To reproduce this bug :

• PR fix: Ensure necessary scripts are loaded into page #210 needs to be merged
• Clone adressbook-portlet example, update liferay-portlet.xml to set /liferay-portlet-app/portlet/instanceableto true in addressbook-form module (to make it possible to put multiple portlet instances on the same page) and deploy it to the Liferay by copying in to {liferay_home}/deploy folder.
• Open localhost:8080 and login into Liferay
• Click on the "Edit" icon to start edit the page
• Open "Fragments and Widgets" panel on the right side of the page, go to "Widget" tab
• Find "Vaadin Sample" category on the bottom and then "ContactForm" element
• Drag and drop the widget to the page.
• Drag and drop the widget to the page one more time
• Check that the second portlet/widget is blank

Expected behavior: All portlet instances added to the page are rendered correctly
Actual behavior: Only first portlet instance added to the page is visible

Versions:
Vaadin Portlet version - 1.0.0.beta4
Flow version - 2.7.8
Liferay version - 7.4
Java version - 11

[mcollovati]

After a quick investigation seems like that when second instance is added to the page, _getClientStrategy is invoked before window.Vaadin.Flow.Portlets.initListenerRegistration, because the webcomponent is already registered and customElements.whenDefined(tag) is immediately resolved.