Whilst if you know the code to use you can add it in a template or exported JS document, web site design is not a part of Tinderbox’s intended feature set. I suggest you find a pop-over method of choice (I don’t think there is a ‘master’ method) test the software in your web page editor then you can work out what code goes in what source note so that Tinderbox creates those files & HTML code on export.
Roll-overs (i.e. triggered via cursor movement rather than a click) can be effected in just HTML, HTML+CSS, HTML+JS or all three. It rather depends on what you want the pop-up to do, whether you want a custom styling, what sort of device is used (e.g. read on a computer screen vs on a phone), etc.
I agree this is in fact mostly a website design question so not specific to Tinderbox even though it is something that may come up consistently when using the software. The the freedom/flexibility is certainly there - anything that generally works in HTML seems to do fine integrated into Tinderbox.
Thanks. Yes, it can seem counter-intuitive to use another tool as well. But HTML editors (some free) have things like syntax highlighting and debugging tools that would simply bulk out Tinderbox.
‘Reversing’ code into a template is a matter of finding what goes where in the template (or $Text) and either inserting actual HTML code or using a relevant Tinderbox Export code to generate desired HTML.