When putting a horizontal rule (
<hr>) inside a spoiler: On the android app, things as links or code blocks seem to be the only thing hidden in spoilers. On a mobile browser those things completely disappear, and when you click to show the spoiler, the normal text disappears.
The following pictures are screenshots of this answer, but I also encountered this problem on other posts.
Stack Exchange Android App (1.0.89):
StackExchange.com on mobile: