Hello,
We have really strange issue after updating one of our test servers to ExtLib V12.
Our application is using Application Layout control that is resides inside a custom control. Facets added to the left and middle pane of the Application Layout control.
This custom control included into the Xpage and actual data controls added into the facets of the custom control.
All worked fine until ExtLib V12 was installed on one of the servers.
Right after upgrade data elements of the form that located in the LeftFacet and MiddleFacet of the Application Layout control became un-clickable - elements are rendered and visible on the screen but they will not respond to mouse clicks. The elements are - navigator, dynamic view and some buttons.
Here is the screenshot of the rendered HTML from firebug on the system with ExtLib v12:
notice div with empty class

Here is the screenshot of the rendered HTML from firebug on the system with ExtLib v11:
notice div with proper class name and one extra div

Here is side by side comparison of the HTML rendering:

Until this resolved we will not be able to use ExtLib V12 unfortunately.