Folding of .ui files / .xml files has disappeared

The plain text editor seems to have lost the ability to fold sub-elements of .ui/.xml files, somewhere since (I think) version 4.8 . After a recent update I can't fold blocks of XML to hide them. This is a real PITA as I've been using the ability to fold and cut and paste blocks of XML in Qt Designer forms to get around that stand-alone / Qt Creator plugin evil feature of not maintaining the items in a QGridLayout in any sort of order which makes for a massive amount of noise in a diff for even a minor change in any source code versioning system such as Git...