On a flat panel, adhesive holds a sheet against one plane. On a curved one it has to hold that sheet through a bend, and at the corners it has to hold it through two bends at once. That corner is where edge-to-edge glass lifts, and it lifts for reasons of geometry rather than glue quality.
Glass does not want to bend in two directions
A flat sheet can be persuaded to follow a single curve — roll a piece of paper and it stays rolled. Ask the same sheet to follow a curve running down the side and another running across the bottom at the same time and it cannot, because the material would have to stretch in one direction and compress in the other. Paper creases. Glass does not crease; it stands off.
That is a corner. The side bend and the bottom bend meet there, and the sheet is being asked to do something the material will not do.
What that does to the adhesive
The glass is trying to spring back to flat, and at the corner it is trying hardest. The adhesive is holding that load continuously, not just at the moment of fitting. Warmth from your hand, a pocket, a summer afternoon all soften the bond slightly, and the corner is the first place that shows as a bright crescent where the sheet has stood off the panel.
A corner lift on a curved phone is almost never a bad batch of adhesive. It is a sheet cut too far into the bend, doing exactly what the physics says it will.
Two ways to answer it
One is to bond with a liquid adhesive cured in place, which fills the curve rather than bridging it. It works, and it is a messy job with a UV lamp that has to be done once and correctly.
The other is to stop the glass before the bend. We relieve the cut at the edge so the sheet covers the flat of the panel and stops short of the contour, on glass that is still flat enough to hold. You get a fine border at the curve that catches the light, and in exchange the corner stays down and a case closes over it without levering it up.
Why we chose the relief
Because the failure mode of the alternative is worse. A sheet that covers the curve and then lifts has an edge standing proud exactly where your thumb and your pocket find it, and it takes dust underneath with it. A sheet that never claimed the curve has nothing to lift.
If your current one is lifting
Do not try to press it back. The bond at a lifted corner has already picked up dust and will not re-wet. Take it off, clean the panel including the curve, and fit a fresh one — and if it lifted within days, the cut was reaching into the bend.
Questions people ask
Does tempered glass work on a curved screen?
Yes, if it is cut for that phone and stops just inside the curve. Rigid glass cannot follow a bend, so a sheet that tries to cover the curved edge lifts at the corners. A slim uncovered border at the sides is what keeps it bonded.
Why does tempered glass not cover the curved edge of my phone?
Because glass that reaches into the bend is being held against its own spring, and at the corners it pulls away within days. Stopping short of the contour is deliberate: the sheet covers the flat face, where most knocks land, and stays down.
How do I know if my phone has a curved screen?
Look at the phone side-on: if the display itself rolls down over the side edges, it is curved. Every phone page on this site lists the edge type in its spec table — flat, 2.5D (flat with a softly rounded glass edge) or curved.
Is a film screen guard better for curved phones?
For covering the very edge, a flexible film can wrap further round the curve than glass can. It gives up most of glass's impact protection to do it. For most curved phones, glass cut inside the curve is the better trade.