Focus for NPAPI plug-in for Alt-Tab

In our NPAPI plug-in, we handle WM_SETFOCUS in the plug-in's wndproc.
We get here when we switch tabs in a Firefox browser window.
But, if we Alt-Tab to another Windows application and come back, we
don't get this event.
If we do Alt-Tab for the second time, this event does get fired!
How does Mozilla code decide when to handover WM_SETFOCUS to plug-in's
wndproc and when not to?
How can we guaranteedly receive this event in this particular Alt-Tab
case when the plug-in containing Firefox top level window is indeed
getting activated?

BTW, I've seen the same behavior for Flash & PDF plug-ins.

Any solutions, workaround?