The edge of the glass has a slight bevel. You are correct that all glass is more sensitive at the edges but it would still take a significant "sharp" impact to crack a shower pane.
The panes we use are specifically manufactured for this use and are typically 3/8" or 1/2" thick.
Tempered glass is actually significantly stronger than normal glass although it is more brittle, which is desirable in this application because we want it shatter into smaller safer pieces rather than shards if it does break.
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