Magic Mouse doesn't select in Mail after El Capitan

After upgrading from Mavericks to El Capitan three weeks ago, I've had several occasions where the Magic Mouse will not select in Mail. For example, I try to select a message in the list with the Magic Mouse and it won't, yet I can go up and down the message list with the keyboard arrow; or, I won't be able to set a flag on the current message using the Magic Mouse. After waiting a few minutes, sometimes going to another application and sometimes not, the Magic Mouse will again work in Mail. The Magic Mouse works fine when going to other applications (leaving Mail open). Anyone have an idea why this is happening, and better still, how to eliminate the problem? This is really annoying... and it never happened in Mail while using Mavericks.

Also, does anyone know how to turn-off the lateral scroll on the Mail message list that shows the option to "Mark as Unread" or "Delete"?

iMac 21.5" Late 2009, OSX 10.11.2