I've got the same problem. Here's the thread I started about it:
https://www.pokecommunity.com/threads/171254
I've come up with a temporary work-around, though. Click the "quote" button on the page you want to read. If all that's inside the spoiler that you want to see is text, you can just scroll down and read it. But if there are images or a video or lots of font/size/color tags that make it hard to read, you can do this:
- delete the {quote} tag (you don't even have to delete the {/quote} tag, because deleting the first one will make the second one invalid)
- delete all the {spoiler} tags (and like the quote tag, you don't need to delete the {/spoiler} tags either)
- And then click on "preview post."
And you can see the whole post as if the spoilers were opened. It's long and inconvenient, but it's better than not being able to read it at all.
Deleting the spoiler tag, obviously, stops it from being a spoiler. But if you don't delete the quote tags as well before clicking "preview post," PC will say "The message you have entered is too short. Please lengthen your message to at least 25 characters." Instead of typing random stuff just to get the preview post to work, it's easier to delete the quote tag.
Just be careful not to push "submit reply." And, unfortunately, if the spoiler you're trying to open already is inside another quote, when you click on the "quote" button
that entire quote (including the spoiler inside it) will be deleted, instead of letting you do a quote-inside-a-quote. But if that happens, 95% of the time you can just scroll up to the original post that was quoted and follow the procedure from there.
If that didn't make any sense, let me know and I'll explain it with screenshots or something. :P