Tempered glass is really only susceptible to shattering if it's hit by something that can scratch or chip the edge glass. Most items in a shower are soft; plastic bottles, elbows, even boney elbows. Metal shower heads can be hard, but they usually don't have much mass behind them, so they'll usually just deflect off the shelf if they collide. Though I don't have glass in my own showers, I've installed plenty of glass shelves in houses. In showers, medicine cabinets, and the one that always makes me cringe: in kitchen cabinets. I give the owners of the homes I've built an annual call. No one has ever told me about a broken shelf. Every few years one of the glass shops has a "let's break glass" demonstration. I was placing an order at the shop several years ago when they had a demonstration. I stuck around to watch, it's pretty amazing how much abuse tempered glass can take. I'm not scared of using tempered glass, but I respect that it still is glass.

Thanks OHG! I definitely didn't do it myself. After seeing all the intricate cuts, not to mention how he had to deal with our not-at-all level, square or plumb house, I can't imagine doing it. The tiles on the wall are 12x24. Our tile guy nicely told me that this was the only bathroom he's ever done where he only used 16 whole tiles in the entire room! Then I handed him a beer to make up for it. :)

You're seeing through the glass to the back wall. Cement board, thinset, etc. If you lower your eyes to be level with the shelf, you might be able to see all three sides of the niche through the thickness of the glass shelf. Had the captured edges of the shelf been fully bedded in thinset, it usually results in a more uniform appearance. With thick glass like that, I'll often paint (usually black but sometimes another color depending on the installation) the edges of glass that will be captured to prevent that from happening.

Really like the build in desk idea (with shelves above) -- but you might want to add a door anyway so you don't have to worry about controlling the clutter -- putting everything away each time you get interrupted. You could use a shallow bookcase as a door over the hidden closet. https://murphydoor.com/products/flush-mount-paint-grade-bookcase-hidden-door?variant=20781346979899&gclid=EAIaIQobChMItZionYLz5gIVCZSzCh1hHw1XEAQYBSABEgK0K_D_BwE Alternately, you could panel that wall and have the door(s) appear to be just more paneling -- creating hidden closet space for however you choose to use it. http://nextluxury.com/home-design/hidden-door-ideas/ Also, would at least consider adding a door sized mural of a natural space to the back wall so you're not looking at a wall while sitting at your desk. https://www.google.com/search?biw=1366&bih=628&tbm=isch&sa=1&ei=JUMVXsbzGuql_QbZt6fABQ&q=door++murals+of+natural+spaces&oq=door++murals+of+natural+spaces&gs_l=img.3...436189.646101..646295...1.0..0.188.3662.50j4......0....1..gws-wiz-img.....0..0j0i131j0i67j0i10j0i5i30j0i30.-mbZPHunQ_Y&ved=0ahUKEwiGmsj-_vLmAhXqUt8KHdnbCVgQ4dUDCAY&uact=5#spf=1578452398881