Whenever you click on one of the post previews that exist on pageload (Answer preview on a question page, previews on /questions/ask and /posts/postId/edit), Stack Exchange will have the preview's textarea steal your browser focus. This can be quite annoying - it can make selecting the rendered text and code difficult, and makes it nearly impossible to run Stack Snippets in longer posts.

This userscript disables the part of Stack Exchange's Javascript that causes this behavior.

Here's a Meta SO discussion of this issue.