Hi CRJO,
I think you will find the Cockpit Windscreens and side Screens, although transparent, have some material in them that nicely shields them from GPS Signals. I don't have experience on the 787 but in the 747 and other series, the Window Heat operates by passing electrical current through a super thin rolled gold film to heat the Perspex/Glass to make it more malleable. I think this is enough to shield portable GPS from signals with W/H on or off from experience.
Most likely this is why previous posts have suggested escape hatch areas etc. are a better place to try getting a signal for a portable GPS.
On your lovely machine, you have external, powered GPS Antennae that do a great job.
Even on some lighter Aircraft and Heli's that don't have Window Heat, it is still sometimes difficult to gain a reliable GPS Signal for portable GPS, where it isn't possible to see vertically up to the sky, or only a partial view.
Hope that helps