Thanks to the input offered by both: Vincent Maverick Durano
, and ZurdoDev
, I found the root of the problem!
It was a div tag inside an updatepanel (Vicent, you are right, they are "Evil"), which simply decided to ignore the Update() method being invoked from code.
divClickOut.Visible = false;
Since the div is transparent a has not effect on tab, etc. key-navigation, its presence was not obvious.
On the other hand, as ZurdoDev suggested, I did use the browser's debug tool (again), to inspect scripts, html, etc, but overlooked the simple, yet effective right-click => "Inspect Element"
These ideas allowed me to detect the problem of this "odd behavior"; now, I'll get on fixing it.
Thank you guys.