It was also shown prominently at the end of Generations, shattered after the crash on Veridian III. So definitely not an indestructible part of the bridge structure. Looks like it was at least an inch thick, possibly more.
View attachment 47589
That always bugged me. From the beginning of the film the various windows on the D provided motivated lighting. (Which was AMAZING!)
Then the bridge is dark during the crash and only lit by fire after the lights go out until they stop and we get this spotlight looking thing. They kind of make it seem like we're only getting light because the window is broken, don't they? It's a WINDOW. It should have been lighting the bridge during the whole descent.
Don't blame me! They made up the rules in the beginning of the movie!
It's much more believable lighting in the last scene when Picard and Riker beam up. Which I love.
OTOH, I just rewatched the crash scene and it. Is. Thrilling.
GEN is the best TNG movie.