The Define Luminaire dialog in AGi32 v.20.x (and later versions) has overlapping and cut-off panes, similar to the image below.
There are a few things that could cause the Define Luminaire Dialog to size/display incorrectly. The solution, hopefully, is easy and requires making a change to the Define Luminaire dialog settings (see below).
Access the "Startup Dialog Size and Controls Configuration" setting, set it to the "Best Fit" option, save the setting as a Startup Default, then click "Ok" (see image below). If you may also be able to try the "Small" option to see if you like the smaller, tabbed layout if the "Best Fit" option defaults to the large-sized dialog and the smaller dialog size with tabs to access the different settings is more to your liking. Either way, the "Best Fit" option should solve the problem (and may end up being the "Small" size on smaller monitors). Just make sure to save it as a Startup Default so the setting is retained.