The editing help page simply says the following with regards to spoiler tags:
To hide a certain piece of text and have it only be visible when a user moves the mouse over it, use the blockquote syntax with an additional exclamation point:
At the end of episode five, it turns out that
he's actually his father.
Everyone knows that, when on a desktop computer, it will reveal the text if you hover the mouse over the spoiler. However I just accidentally found out that, after hovering and revealing the spoiler, if you click on it once then the spoiler remains revealed even if you move the mouse away!
Is this a brand new feature? Or has it always been like that?