One issue I have when replying to emails is that I lose visibility to the message I am replying to (sometimes I’ll need to verify exactly what was said, or to quote a portion of the reply in my own response).
The actual reply gets pushed to the top or bottom of the screen, and I always have to scroll around endlessly just to re-read or copy exact text from the message.
This happens in desktop browsers and on mobile devices.
On the desktop, notice the actual message that appears in the reply textbox, versus the entire rest of available screenspace:
Very little room is dedicated to the message content that you are replying to.
If I decide to look at the message in the textbox (bottom), I have to scroll down, meaning I might lose focus of where I was typing and have to replace the cursor. Same if I decide to look at the message towards the top of the page, I lose focus of where I was typing, and have to perform needless scrolling.
On a mobile device (iOS pictured below), the only way to get to the message is by scrolling downward, possibly causing a loss of cursor focus.
Imagine having to perform this up-and-down action for every email you reply to on a daily basis.
This is very inefficient and time-consuming, especially when I spend most of my day in my inbox.