Since installing El Capitan on 2009 iMac, in Apple Mail, a selected message contents does not scroll in the bottom window when the mouse scroll wheel is used, or when the down arrow key is pressed. Restarting Mail, rebooting does not change the issue.
Using latest version of USB Overdrive extension, but disabling this does not fix the problem.
I saw a proposed solution in Apple Discussions wherein deleting com.apple.mail.plist "in /Library/Containers" (?)solved the problem for one user, but I have no such plist in that folder.
Anybody having this problem?