Open Tags before <!--nextpage--> impacts entire webpage

[crushgear]

If a chunk of your post is formatted (like strong or em) and you insert the <!--nextpage--> tag after an open formatting tag, it will actually format the rest of your entire page (comments, sidebar, etc.) with the tag.

I was able to replicate this bug on trunk in both Chrome and Firefox.

Here's an example of content that would trigger this bug:

<strong>Testing the nextpage bug <em>

<!--nextpage-->

tag bug. </strong></em>

[johnbillion]

Sounds like we need to run force_balance_tags() on the content that comes before <!--nextpage--> when it's displayed on its own.

[SergeyBiryukov]

Duplicate of #6297.