Steps to reproduce the problem:
- disable the wrap around option for the simple search (Ctrl+F)
- place the cursor at the end of the document
- search for something that exists in the document

The search entry background becomes red, because the text is not found after the text cursor. But since the text exists in the document, it should be possible to go to the previous occurrence. The problem is that the forward and backward buttons are insensitive when the search entry has the not-found state.

So the simplest solution is to always set the forward and backward buttons as sensitive, even if the action does nothing.