On an unmodified Game Boy Pocket, a small round ~0.5mm diameter bubble appeared on the center-left side of the screen eight months after I had last used it (there was no indication of it previously). What might be the cause of such a bubble (to avoid it happening again), and how does one go about fixing it (without having to replace the whole LCD screen if possible)?
The pixels on the display seem to work well, it seems to be the plastic film right above the display. If I tilt the Game Boy at a certain angle, it seems that the area around the bubble is slightly darkened. The bubble, which looks like a drop of water, does not seem to be darkened by looking at it directly. By using a camera, though, the whole area seems to be darkened, including the bubble.
The unit was safely stored the whole time in its original protective hard-shell plastic casing, horizontally lying down with its screen upward inside closed wooden furniture in my living room. Could the screen's plastic film be deteriorating due to its age?