We have developed a UXP plugin for InDesign with a Webview in a panel. The Webview content was sending a message to the UXP plugin using window.uxpHost.postMessage(msg). (msg being replaced with content to be handled and merged into a document by the UXP plugin.) In InDesign 21.3, window.uxpHost was visible to the Webview content and the Webview content was able to post a message to the UXP plugin, but after upgrading to InDesign 21.4, the Webview content cannot detect window.uxpHost and the message can no longer be transmitted.

To reproduce, create some web content that will post a message using window.uxpHost.postMessage(msg) based on some interaction in the web content. Embed that content in a UXP plugin panel for InDesign with a listener for the message. Ensure required permissions for the webview including localAndRemove messagebridge and applicable domains are defined in the plugin manifest. Launch plugin in InDesign 21.4. Trigger the message send from the web content. No message will be received by the listener.