Heavily discussed and diagnosed in this thread: [Link Removed]
[Link Removed] narrowed down the cause:
It's the calculation inside the SWindow that is wrong, sometime. It adds SWindowDefs::DefaultTitleBarSize in SWindow::GetWIndowSizeFromCientSize
I am not sure how to prevent that in all the cases.
RESULT: There's unexpected white space at the bottom of the window: [Link Removed]
EXPECTED: The window fits its contents tightly, without the added whitespace at the bottom: [Link Removed]
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-166960 in the post.