This has happened to me several times, but I do not know how to recreate the problem.
Situation: In Standard 8.5 Notes mail template, when working once in a while I find that the clicks in the scroll area are not recognized. I can select any message in the standard inbox view, but clicking on the window controls to scroll up/down/pageup/pagedown/dragthumb have no action. I can use the keyboard up/down keys and they work, but the window controls do not.
I cannot make this occur reliably. It generally happens when working in mail for 4-5 hours.
Work-around is to close inbox tab and re-open mail. Switching to another application has no effect.