After updating to 4.3, I cannot properly open the 4.2.1 version of my projects (NOT the 4.3 version, of course) in an older version of the editor.
They launch properly and everything functions as expected, but trying to open windows (i.e. clicking on a blueprint to edit it) doesn’t actually CREATE those windows. It becomes impossible to edit BPs, materials, etc.
I assume this is because the 4.3.0 install overwrote some critical file related to the handling of windows, as I can open BPs and the like in 4.3 just fine.
Actually, the problem is solved. What had happened was, for some reason, the secondary window (that pops up for editing bps and the like) had been minimized and moved off-screen. Using right-click > move in Windows to grab it I was able to get it back to a usable location. No idea what caused this to happen, but there you go.