Yes, you are right, this is Becky Internet Mail :)
I have been using this program for many years, and despite the minor drawbacks, I love this small and fast email client.
Thank you for suggesting a way to access message headers.
I tried to force focus directly from the event_loseFocus() method, but it's a bad solution... and it didn't work.
But still I don't understand why the focus does not fall on the headers when navigating with the Tab key...
because these objects have the following attributes:
isFocusable == True
For me this is a strange and incomprehensible behavior... I just faced exactly the same situation in another program.
Lukasz, could you share your add-on for Becky?
Thank you and good luck!