I see one flaw in the design of the reply interface.
It is the fact that after hitting "Reply to Thread" this button does not disappear or fade out, but remains there and is visually the most prominent one. At the same time it becomes a trap for users: its sole function in the reply mode is to erase the whole content of the typing area. I do not know any "Undo".
To the contrary, "Post Quick Reply", the one the user would naturally prefer to use, is small and relatively hard to spot. Therefore it is not so difficult to use "Reply to Thread" instead of the intended "Post Quick Reply", causing often the loss of a significant amount of work. I fell victim to this button many times and would recommend disabling it when the user is already typing the reply test.