When replying to a long comment, the text entry box and buttons get pushed off the bottom of the screen, and it doesn’t seem to be possible to scroll down here. In the attached screenshot, only the top of the box is visible, and it disappears entirely when the keyboard is up.

On a vaguely related note, I would also like the text of the parent comment to be selectable on this screen for ease of quoting, rather than having to back out to the thread and copy the whole text.