I do understand the complexity involved, however believe this to be a particularly nasty bug that could go unnoticed in testing and potentially cause data loss.
I have written the following jQuery to select the first visible input button or textbox (that isn't a radnumeric) and have assigned it to OnClientMouseOver. This is not ideal as hovering over the tabstrip will move the focus to another control, however it appears to be too late to use onclienttabselecting as the incorrect behaviour occurs. I do not see a onclientclick in the API doco.
I suppose that I could update the below function to then refocus the initial item
I assume that this has been logged and may be looked at in a future hotfix, or do you require me to submit this via your issue tracking system?