When using the AJAX web client on any OS/Browser combination focus is lost when initiating a reply to a message.
When replying to a message the focus starts in the message pane and then is lost in the process of loading the child page. So that when loading is complete the focus is undetermined. Pressing the Tab key cycles the focus to the Send or in the reverse (Shift + Tab) ends up at the Subject line.
Release 6.0.10_GA_2692.RHEL5_64_20101215170845 CentOS5_64 FOSS edition.
I have seen this since we upgraded to 6.0.10.