Re: In 99% of my conversations people always write only on top of the quoted message.
Yes and you can totally do that, please see my screenshots above.
It's not necessary to expand the "( ... )" to make sure that the original (quoted) text is sent. It will be, unless yo delete the "( ... )".
Now yes I understand sometimes you (and others) would like to expand the "( ... )" to look something up.
Yes in this case we make a new line to have a predictable place where we can put the cursor.
I already wrote this maybe a week ago, and @mikeone linked (above) to and quoted from my responses.
Yes it's somewhat unfortunate that we aren't able to keep the cursor position exactly where it was before you tapped the "( ... )".
But at the same time - I really don't think that everyone needs to expand the "( ... )" (to look something up) every single time, with every single reply that he/she ever writes.
When Gmail / web (which I use a lot) started doing the "..." (maybe a year or two ago), it took some getting used to. Now I love it because it keeps the reply's compose area clean and simple.
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BTW - Gmail (for Android) moves the cursor to the very top of your message, which can be quite far from where you intended to be, farther than you end up in Aqua Mail.