Flickering at the mail preview

I have just updated to the latest beta 5869 I have noticed that the preview is flickering. I found out the cause so that this should reproduceable. When you have an email with a long subject line that goes to the edge of the window, then a flickering happens. It is caused because the UI cannot decide if the last word in the line should be in an new line or at the same.
When you resize the Mailmate window, so that Subject text reaches the edge of the window, there is a sweet spot when this flickering happens.
I have not noticed this issue in the previous versions, so I think this issue must be new.

Update: I have this issue also in the compose window. I have the message preview enabled so that the preview is shown at the right column. It happens when the preview text is right to the edge, that a flickering happen, because the last word on the line is shown in the same line and then in the next line and so on.

Update2: You can find a screen recording video flickering here: https://cloud.ciqua.net/s/G6qYaF5Gs8DBcm9

Cheers,
René