Expandable comment box
The comment text box (which appears when I tap on Add Comment) only shows a maximum of three lines, automatically expanding from 1.5 to 2 to 3 lines.
My suggestion: Make the box user-expandable. It can still expand to three lines automatically if too small, but the top of the comment box (the line stating "Comment on X's question/answer: ... x characters left") could be a grab-bar that can be grabbed and pulled with the finger to expand the box further.
Rationale: I often write long comments and I don't write them linearly. I tap on the text and move the cursor around with my finger and edit them. It's difficult to do this comfortably when the text box is too small, as the cursor would jump great distances easily when I only want it to move to the next line.
Side note: The font size could be slightly bigger, or made bigger when box is expanded more than 7-10 lines. Also, to indicate the grab bar, a faint double-line could appear below the text. The double-line could become fully visible (fading out the text) when the bar is grabbed.
Hiding or closing comment box
There is a "down" arrow on the left side of the comment box, the purpose of which is to discard the comment. When tapped on it, a confirmation message appears.
Suggestion: I suggest changing the "down" arrow to a "close" symbol. Additionally, a "down" arrow could appear below the close symbol when the comment box has been manually expanded enough to give it enough space to appear. This new "down" arrow could simply slide down the comment box and hide most of it except the grab bar. The user can tap on the grab bar to return the comment box to the original size, or grab it and drag it up to a new size.
Rationale: The close symbol better conveys the meaning of discarding the comment and closing the comment box. With the re-purposed down arrow, the user can comfortably read the question/answers while still drafting the comment. Along with the expandable comment box, the down arrow will make commenting an easier and better experience for the user.
Side note: The confirmation message has two negative buttons: Nevermind (not really negative, but negative-sounding; and it's a long compound word) and Discard. I think it is better to have one negative button (Discard) and one positive button (Keep). I also think the Discard button should be on the left, as most users are right-handed and it maybe easier to hit the button on the right.