Any keypress in a hst:template/hst:script property text field in the CMS Console causes a loss of focus

This is a very annoying bug making editing hst:script through the Console pretty much unusable.
NB: this only occurs in a hst:script text field, not in for example a hippostd:content (html body) field.
The hst:script field has a different editor (Freemarker), so likely the cause has to be found there.
Also, this is a regression from 7.9 where this problem doesn't occur.