Great, thanks! Seems to work pretty much as expected. Because of the user-select: none, I can’t actually highlight the spoiler text directly. So it seems like I can only quote spoiler text when I highlight something before it and then drag my mouse over the spoiler text. This means that I can’t highlight spoiler text if the post starts with it (highlighting the text after it and dragging my mouse over the spoiler text from there doesn’t seem to work).
The “ideal” version I mentioned above with partial highlighting of spoiler text also doesn’t work of course, but I didn’t expect it to given the user-select: none situation. While that would be nice, it’s certainly less important than just getting the spoiler tags in general. I do wonder how the spoiler feature would work without the user-select: none, but I wasn’t able to test it just by removing it in the console since the JS code adds it back right away.
Anyway, I just wanted to give some follow up information from my brief testing. Thanks again!