OK I'm back finally with a bit more info on this.
Still trying to track it down and making little headway.
RE: the IFRAME, I changed the css code for the iframe in my 'container' html document to include: iframe { background-color: #ffff99; }
This made the "background flash" for the IFRAME ONLY show a kind of dull yellow (ffff99) but ONLY WITH THE LOGIN PAGE for RC, strangely.
You can test that yourself on our page right now even without a login for our webmail system.
The further STRANGE thing, however, is that this YELLOW does NOT show as the "white flash" for the IFRAME any more once you're logged in -- after successful login to RoundCube, the white flash background is BACK again, same as before.
However, I don't see it when RC is running full screen, i.e. outside of my IFRAME. So Skaero perhaps is right that it's just an iframe thing mostly... but kind of NOT too, either, since the full RC running (vs just login page) does not seem affected by the iframe background color.
Aieee.... this is such a dumb small thing but driving me batty. Any more help, anyone? Please comment if you have ANY ideas out there...
-JC