updated to 4.3 a few days ago and noticed a few UI issues that caused me to go back to 4.2.
Issues i found were the following:
clicking on UI buttons such as the local/world orientation, widget swaps and snapping tools.
Had the error of not showing any options via dropdown list.
Right clicking in BP’s event graph to search for nodes did appear either.
sounds like a window popup animation error of some kind
Hope this helps 
Hi Twibbz,
Thank you for your report. This is a known issue with Window Animations in 4.3. You can find more information about it here: 4.3 Preview > File menu and others stopped working - Programming & Scripting - Epic Developer Community Forums
We are working on solving the issue for a future update. Please let us know if you encounter any other unexpected behavior.
Cheers!
This is a pretty old post but the issue is still relevant as of 4.26.0 so thought I would bump it anyways