As mentioned here, ReaImGui shows empty windows if the ui_scale is set to something different from 1.0.

Though I can work around this issue by leaving it at 1.0 and set UI scaling in the Reaper preferences, Reaper does not scale the fonts with that and sucks ATM when defining larger font sizes in libSwell.config. The only "good" way to have a properly scaled UI in Reaper appears to be setting ui_scale in reaper.ini.

This is how it looks on my system:

So I kindly request ReaImGui to make proper use of the ui_scale value.