Hey, guys. I don't know if this bug has been discussed before, I couldn't find any details about it.
So, when you start typing a reply to a thread on the iPhone or iPad, the preview of your post starts to fill up and the composer simultaneously goes up line by line, hiding behind the upper border of the screen. As a result, you don't see the lines that you wrote earlier.
You can replicate the bug on any iOS device or in the Firefox responsive design mode choosing any iOS device.
Maybe it can be easily fixed with some CSS, but so far I couldn't find any solution.