Hi Chip,
I have a 6 Pro, but fortunately haven't had to open it up yet, but I did work on my Pixel 2. I've gone over the screen replacement guide and there are only a couple of possibilities I can see.
First, was there anything unusual about the replacement? Did any step not go as described in the guide? As far as the screen sticking out, the only possibility I can come up with would be if you had separated the plastic frame that's mounted on the display and left it on the phone instead of coming off with the screen. I'm not giving that a high probability of being the answer because the guide tabs on the frame wouldn't have any place to go into since the old frame would already be filling the holes they're supposed to go into. So unless you cut those tabs off the replacement frame, then that's not likely to be the problem.
I'm assuming you followed the guide and made sure to clean all the old adhesive off the frame before mounting the new screen with new adhesive. If not, I suppose it's possible the combined thickness of two layers of adhesive might account for it, but that seems like a stretch too.
My next thought was that it may be a screen from a manufacturer that was different from the original, but iFixit only sells genuine Google Pixel parts. As nearly as I can tell, the screen is most likely manufactured by Samsung, although I've seen a few posts that indicate LG has made screens for the Pixel line, but nothing that indicates they made them for the 6, though.
Does the screen sit securely on the frame? Is there any gap between the edge of the glass and the phone's frame? If it seems to be secure and there's no gap, then I might be tempted to chalk it up to a possible change in manufacturers with slightly different dimensions; I know that's common on aftermarket iPhone screens, so it's possible Google has multiple sources for their displays.
The only other thing you can do is open it back up (have another adhesive sheet on hand, of course) and see if you can find anything that looks like a possible source of the issue. Perhaps some adhesive got down inside the mounting tabs that's keeping them from seating fully, for example.
However, if the screen is secure and seems to be sealed well, I would personally be inclined to leave well enough alone, unless it's really bugging you and you just can't live with it.
Good luck; let us know what you decide.
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