So, re: file links in NV:
"External Links" that it knows about, such as https etc. are handled properly.
"Internal Links", like file:///, have the behavior described below.
<a href="file:///X:\MyPluginsPy3\_ris_import_dir\abstract.txt">NV Test File URI</a>
When clicked in NV, the .txt file in the link above is opened by NV directly within the NV qtextbrowser widget, and shown in it. In NV, directly. Clicking "refresh book" reloads the current Custom Column text.
If the file extension were, for example, .xyz, and there were no default file handler for .xyz files (in Qt), NV would do nothing but show an error message in DEBUG.
Example: "QTextBrowser:
No document for file:///X:/MyPluginsPy3/_ris_import_dir/test4.ris"
Supposedly the errors is issued because .ris is a random extension that Qt has no standard file handler for. Unlike .txt files.
I will continue researching. Maybe I can allow a user to update on-the-fly what extensions are registered by QDesktopServices, which is what does the work for links. I don't know enough yet to know, and I don't know what I don't know.
DaltonST