The problem is just what the tittle describes: when there are many replays to an email, which are not shown on the list because they are moved to the right of previous email, one has to resize the message list panel to reach last emails.

Also, while resizing this panel, a scrollbar blinks below the list, put it disappears almost immediately. 

Since English isn't my native language, I'm afraid I'm not being clear, so I'm attaching a screenshot.

Reproducible: Always

Steps to Reproduce:
1. Send and receive many emails replays to an email
2.
3.
Actual Results:  
Messages are accumulated below the original email, moved to the right. If the message list is narrow enough to not show last messages, there isn't any horizontal  scrollbar

Expected Results:  
There should be an horizontal scrollbar