The text in the comment box, in the off-topic selection area of the close pop-up; option other, will not disappear or be disabled upon text entry:
I think it is best to have something we have already in place with the comment box:
Your text entry is expected to be mandatory and supplemental (and in my case it's always has been so) to the sentence stub.
The comment box, on the other hand, is optional and hence the placeholder text to guide. I'm not sure I see the reason why the predefined offtopic comment should be changed to placeholder type?