Yes, it is possible to write a plugin that gives extra information in a popup window.

I believe that a good example project might be the TagLEET plugin, with the original source code here or @Michael-Vincent’s fork here – although that’s a more-complicated popup window than the one you had in your screenshot.

I believe a simpler version could be implemented using the Scintilla feature “calltips” (where Sctintilla is the code library that Notepad++ uses to implement the editor portion of Notepad++, so you have access to all Scintilla features in your plugin.)