[MlMt] Option for windows to share screen space, instead of appearing one on top of the previous one

[José Antonio M M]

When a new window appears for writing a new message, it puts itself on 
top of the mail app’s main window. The user then has to put both 
windows side by side when needing to check previous emails. Wouldn’t 
it make sense to have an option for the new window to appear next to the 
previous one, sharing the screen space?