Mail: Compress flat thread indentation
We can have very long email threads (100+ emails).
For each new message, the indentation increases in the message list pane.
The problem is that after a large number of emails in the same thread, the messages disappear from the view, because the indentation with white space eats up all the space in the pane, and one then requires cumbersome right-left-scrolling to locate messages in the thread.
To overcome this problem, one can:
- add support for customizing the indentation (one could set it to zero or one single pixel)
- add a maximum indentation level to 70% of the message list pane