I really like the WYSIWYG compose window on a mobile browser: once activated, the whole mobile phone screen is used: lots of space to compose and re-read, no distraction.
On a computer it is not so: the simple text window takes two thirds of the screen height (in 100% zoom, i use rather 90%; while half-width, full-height would be preferable), the WYSIWYG window is actually even thinner (6 paragraph lines ~ 10 lines in one paragraph). The other drafts do not need to be displayed while composing a mail, these could be hidden for the moment.
It is not so but could be: the compose window could occupy the full available size (or at least height) to allow to display as much of the mail being composed, and possibly even the mail being answered.
My 5 cts…