Your suggestion to use a default window partially worked.
I accessed the "Editing" window.
When I returned to my custom project window, drag and drop worked again.
But when I reloaded Premiere, drag and drop failed again. I reloaded a default window and switched back to my custom window and drag and drop worked again. I guess you have to go through this step each time.
I have 3 external monitors. I was able to drag clips into the timeline and use keyboard shortcuts, but was unable to drag clips or files into my project window. I also upgraded to Catalina OS V 10.15..7
save your preferred workspace and restart Premiere Pro. Now whenever you open Premiere, just close the floating panels and choose from the menu: window > workspaces > reset to saved layout (alt+shift+0). This way you will restore your layout, and the panels will function as it should