I'm pleased yours disappeared. Mine which is identical in both the place and shape has been on my G3 since I got it back in July.
It still gets darker the longer you have heat build up on the screen and when it's cooled down its faint again.
Sorry guys, I forgot to come back here with an update.
Mine eventually disappeared last week after nearly 3 months.
I believe it is the glue.
Someone told me to try leaving the phone under UV lighting. I left mine in direct sunlight for when it was not being used. Seemed to cure it in a few days.
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