It uses the Windows API from what I remember. I’m still having no issues reproducing the dragging with both Chrome and Mozilla, about the same frequency as UE4. I’ve run into the black rectangle issue once more. I’ll put a bug in for that and be sure that I’m the one who picks it up after the fix, so that I can be sure it gets tested enough since the repro rate is so low.
I also managed to come across another bug while testing this, that can be found here: UE-34895 I’ll reply shortly with the bug for the black rectangle issue.