When you invoke some popup in SO site -> Documents -> DocLib portlet (for example Add -> Basic Document), popup renders correctly, but when you submit some invalid data to server – I tried to upload document larger than 100MB on in.liferay.com – it results to plain markup being rendered in browser (EXCLUSIVE portlet URL is used to submit the JSP form in the popup). When no error occurs (most cases), redirectURL is used after processAction, so the response is correct (full portal markup).
Steps to reproduce:
(1) login to SO 2.0
(2) go to SO site -> Documents (where you have rights to add docs)
(3) invoke Add -> Basic Document
(4) enter invalid data, resulting in failed validation on the server side (too large document, I tried 200MB on in.liferay.com, but default is 3MB, which is even easier to exceed)
(5) save and wait till document is uploaded and browser reloaded
Plain markup is rendered with error in the top. Caused by EXCLUSIVE portlet url used to submit the form in the popup.
Stock DocLib does not suffer from this, as it does not use popups for operation in DocLib portlet (uses full portlet URLs). This is closely related to
SOS-1233 (another example of easily failing popup form). I suspect many other popup rendered in SO's DocLib will suffer from the same.