Changing the <UI><Type> field in the manifest.xml from Modeless to Panel for an extension which has already been run as a Modeless type causes InDesign to crash, or fail to display the panel. This change was possible in previous versions of the application, and works correctly in Illustrator.
Steps:
1) Start with the UI Showcase sample from the Adobe CEP resources GitHub. https://github.com/Adobe-CEP/Samples/tree/master/UI_Showcase
2) Install it
3) Start the InDesign application
4) Open the Modeless version of the UI Panel from the Extensions menu
Result: this shows correctly
5) Close the panel, shut down InDesign
6) Change the UI/Type for the Modeless type to Panel in the manifest.xml
7) Start InDesign
8) Select the Modeless version of the UI Showcase from the extensions menu
Result: Nothing happens, or InDesign crashes
Expected: Should show the UI Showcase in a panel