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After some googling I found that without a support contract it was advised to put bug reports here. So here goes!
I have SQL developer 18.4 on windows 7 and mac mojave and this issue happens for both. If I open the popup for a large text field from the query results the default settings properly centers this popup. But since some of my data is pretty big I tend to increase the size of this popup, which afterwards causes this popup to not be centered anymore: It simple starts at the same x/y coordinates from the top left corner. The code could be made a bit smarter to properly center this popup, regardless of the size.
(This issue exists from at least version 4 in 2016.)
I've attached some anonymized screenshots to demonstrate, before and after I set the new size of the popup.