When the number of modules in a particular category becomes large enough, there is no blank space at the bottom of the list in which to right-click so that you can add another module.
The workarounds are not very pleasant:
Maximize the browser window and reduce its zoom level until there is empty space at the bottom of the list.
Manually edit config.json
Very carefully find the one pixel of margin at the bottom of the list where you can always right click.
Suggestion: add a right-justified "+" (or perhaps a "⋮") menu button to the collapsible section headers.
When the number of modules in a particular category becomes large enough, there is no blank space at the bottom of the list in which to right-click so that you can add another module.
The workarounds are not very pleasant:
config.jsonSuggestion: add a right-justified "+" (or perhaps a "⋮") menu button to the collapsible section headers.