Cannot Identify header line text at mouse pointer after mouse drag to different frame

[Marc Munro]

Emacs 26.2:

I am trying to identify the header line text at the mouse position at
the end of a mouse drag.  

If I drag from a header line to the same header line, or to another
header line in the same frame, all is well, but if I drag to a header
line in a different frame the posn-xxx functions appear to not realise
the windows in that frame have header lines.

My header lines contain a number of buttons, and I'm trying to
implement drag and drop functionality between them.

Interestingly, during a drag operation when I mouse over the buttons in
windows within my start frame, all of the tooltips and face changes
associated with the buttons work just as they do outside of drag
operations, but in other frames the header line appears inert.  If I
continue dragging back to the source frame, the header line buttons
that I mouse over continue to interact normally.

It looks like emacs is unwilling to acknowledge the header lines of any
but the source window while mouse dragging is under way.

Any ideas?  Is this a deficiency, a philosophical issue (ie, it has to
be like this because...), or am I missing something?

__
Marc