There’s a couple of things that cause my context menus to both go black and also to flicker. FWIW I have an RTX 3080 on updated drivers, using UE 4.26.2.
First, if I have anything fullscreen on my second monitor (videos playing through Firefox, or watching something in VLC, for example) while I work in Unreal on my main monitor, after about 15-20 minutes, the context menus will simply stop drawing anything except black boxes when you open them. I have to restart the editor to temporarily fix this. So obviously the stupid long-term fix is to simply not watch anything in fullscreen on my second monitor. Not great.
Second, the cause of the incessant flickering for me where context menus were completely illegible and unclickable was enabling “Enable Window Animations” while also using a second monitor. The only way to interact with the menus then would be to bring focus to another window offscreen, then directly click onto File or Edit in Unreal, and interacting with the context menu (and by extension Editor Preferences window) while focus is not on Unreal. Clicking to expand these menus would not bring focus to the Unreal window, allowing me to change things. Obviously that’s not a serviceable workflow at all, so I disabled Enabled Window Animations completely to fix that problem.
In both cases the problem seems to be Unreal context menus do not like multi-monitor setups at all, and things break.
You would think every developer at Epic use multiple monitor setups, but maybe none of them activate window animations, or none of them watch something fullscreen on their second monitor, or whatever else may be strangely unique about a seemingly common configuration. I only have two screens, and my secondary monitor is on my left.