This has been happening for me for AGES, but I have worked around it and sort of ignored it. Until someone else mentioned it was happening to them too!
Everytime I open a trade window.. it is oversized (length) and covers some of my bags. Once I put something INTO the trade window that it pops back to normal. I know I'm not the only one with this issue.
Since I do hundreds of trades a day, it can get a bit annoying. I've had to even move my trading bags over to either one side of the other just for convenience. It's just something a find a little annoying.
--Jess